Added BETA api for Tab Bar/Tabs widgets. (#261, #351) (merged this feature from the from Docking branch so it can be used earlier as as standalone feature)

- Added BeginTabBar(), EndTabBar(), BeginTabItem(), EndTabItem(), SetTabItemClosed() API.
- Added ImGuiTabBarFlags flags for BeginTabBar().
- Added ImGuiTabItemFlags flags for BeginTabItem().
- Style: Added ImGuiCol_Tab, ImGuiCol_TabHovered, ImGuiCol_TabActive, ImGuiCol_TabUnfocused, ImGuiCol_TabUnfocusedActive colors.
- Demo: Added Layout->Tabs demo code.
- Demo: Added "Documents" example app showcasing possible use for tabs.

imgui.h

@@ -135,6 +135,8 @@ typedef int ImGuiFocusedFlags;      // -> enum ImGuiFocusedFlags_    // Flags: f
 typedef int ImGuiHoveredFlags;      // -> enum ImGuiHoveredFlags_    // Flags: for IsItemHovered(), IsWindowHovered() etc.
 typedef int ImGuiInputTextFlags;    // -> enum ImGuiInputTextFlags_  // Flags: for InputText*()
 typedef int ImGuiSelectableFlags;   // -> enum ImGuiSelectableFlags_ // Flags: for Selectable()
+typedef int ImGuiTabBarFlags;       // -> enum ImGuiTabBarFlags_     // Flags: for BeginTabBar()
+typedef int ImGuiTabItemFlags;      // -> enum ImGuiTabItemFlags_    // Flags: for BeginTabItem()
 typedef int ImGuiTreeNodeFlags;     // -> enum ImGuiTreeNodeFlags_   // Flags: for TreeNode*(),CollapsingHeader()
 typedef int ImGuiWindowFlags;       // -> enum ImGuiWindowFlags_     // Flags: for Begin*()
 typedef int (*ImGuiInputTextCallback)(ImGuiInputTextCallbackData *data);
@@ -528,6 +530,14 @@ namespace ImGui
     IMGUI_API void          SetColumnOffset(int column_index, float offset_x);                  // set position of column line (in pixels, from the left side of the contents region). pass -1 to use current column
     IMGUI_API int           GetColumnsCount();
 
+    // Tab Bars, Tabs
+    // [BETA API] API may evolve!
+    IMGUI_API bool          BeginTabBar(const char* str_id, ImGuiTabBarFlags flags = 0);        // create and append into a TabBar
+    IMGUI_API void          EndTabBar();
+    IMGUI_API bool          BeginTabItem(const char* label, bool* p_open = NULL, ImGuiTabItemFlags flags = 0);// create a Tab. Returns true if the Tab is selected.
+    IMGUI_API void          EndTabItem();                                                       // only call EndTabItem() if BeginTabItem() returns true!
+    IMGUI_API void          SetTabItemClosed(const char* tab_or_docked_window_label);           // notify TabBar or Docking system of a closed tab/window ahead (useful to reduce visual flicker on reorderable tab bars). For tab-bar: call after BeginTabBar() and before Tab submissions. Otherwise call with a window name.
+
     // Logging/Capture: all text output from interface is captured to tty/file/clipboard. By default, tree nodes are automatically opened during logging.
     IMGUI_API void          LogToTTY(int max_depth = -1);                                       // start logging to tty
     IMGUI_API void          LogToFile(int max_depth = -1, const char* filename = NULL);         // start logging to file
@@ -760,6 +770,31 @@ enum ImGuiComboFlags_
     ImGuiComboFlags_HeightMask_             = ImGuiComboFlags_HeightSmall | ImGuiComboFlags_HeightRegular | ImGuiComboFlags_HeightLarge | ImGuiComboFlags_HeightLargest
 };
 
+// Flags for ImGui::BeginTabBar()
+enum ImGuiTabBarFlags_
+{
+    ImGuiTabBarFlags_None                           = 0,
+    ImGuiTabBarFlags_Reorderable                    = 1 << 0,   // Allow manually dragging tabs to re-order them + New tabs are appended at the end of list
+    ImGuiTabBarFlags_AutoSelectNewTabs              = 1 << 1,   // Automatically select new tabs when they appear
+    ImGuiTabBarFlags_NoCloseWithMiddleMouseButton   = 1 << 2,   // Disable behavior of closing tabs (that are submitted with p_open != NULL) with middle mouse button. You can still repro this behavior on user's side with if (IsItemHovered() && IsMouseClicked(2)) *p_open = false.
+    ImGuiTabBarFlags_NoTabListPopupButton           = 1 << 3,
+    ImGuiTabBarFlags_NoTabListScrollingButtons      = 1 << 4,
+    ImGuiTabBarFlags_FittingPolicyResizeDown        = 1 << 5,   // Resize tabs when they don't fit
+    ImGuiTabBarFlags_FittingPolicyScroll            = 1 << 6,   // Add scroll buttons when tabs don't fit
+    ImGuiTabBarFlags_FittingPolicyMask_             = ImGuiTabBarFlags_FittingPolicyResizeDown | ImGuiTabBarFlags_FittingPolicyScroll,
+    ImGuiTabBarFlags_FittingPolicyDefault_          = ImGuiTabBarFlags_FittingPolicyResizeDown
+};  
+
+// Flags for ImGui::BeginTabItem()
+enum ImGuiTabItemFlags_
+{
+    ImGuiTabItemFlags_None                          = 0,
+    ImGuiTabItemFlags_UnsavedDocument               = 1 << 0,   // Append '*' to title without affecting the ID, as a convenience to avoid using the ### operator. Also: tab is selected on closure and closure is deferred by one frame to allow code to undo it without flicker.
+    ImGuiTabItemFlags_SetSelected                   = 1 << 1,   // Trigger flag to programatically make the tab selected when calling BeginTabItem()
+    ImGuiTabItemFlags_NoCloseWithMiddleMouseButton  = 1 << 2,   // Disable behavior of closing tabs (that are submitted with p_open != NULL) with middle mouse button. You can still repro this behavior on user's side with if (IsItemHovered() && IsMouseClicked(2)) *p_open = false.
+    ImGuiTabItemFlags_NoPushId                      = 1 << 3    // Don't call PushID(tab->ID)/PopID() on BeginTabItem()/EndTabItem()
+};
+
 // Flags for ImGui::IsWindowFocused()
 enum ImGuiFocusedFlags_
 {
@@ -956,6 +991,11 @@ enum ImGuiCol_
     ImGuiCol_ResizeGrip,
     ImGuiCol_ResizeGripHovered,
     ImGuiCol_ResizeGripActive,
+    ImGuiCol_Tab,
+    ImGuiCol_TabHovered,
+    ImGuiCol_TabActive,
+    ImGuiCol_TabUnfocused,
+    ImGuiCol_TabUnfocusedActive,
     ImGuiCol_PlotLines,
     ImGuiCol_PlotLinesHovered,
     ImGuiCol_PlotHistogram,
@@ -1003,6 +1043,7 @@ enum ImGuiStyleVar_
     ImGuiStyleVar_ScrollbarRounding,   // float     ScrollbarRounding
     ImGuiStyleVar_GrabMinSize,         // float     GrabMinSize
     ImGuiStyleVar_GrabRounding,        // float     GrabRounding
+    ImGuiStyleVar_TabRounding,         // float     TabRounding
     ImGuiStyleVar_ButtonTextAlign,     // ImVec2    ButtonTextAlign
     ImGuiStyleVar_COUNT
 
@@ -1114,6 +1155,8 @@ struct ImGuiStyle
     float       ScrollbarRounding;          // Radius of grab corners for scrollbar.
     float       GrabMinSize;                // Minimum width/height of a grab box for slider/scrollbar.
     float       GrabRounding;               // Radius of grabs corners rounding. Set to 0.0f to have rectangular slider grabs.
+    float       TabRounding;                // Radius of upper corners of a tab. Set to 0.0f to have rectangular tabs.
+    float       TabBorderSize;              // Thickness of border around tabs. 
     ImVec2      ButtonTextAlign;            // Alignment of button text when button is larger than text. Defaults to (0.5f,0.5f) for horizontally+vertically centered.
     ImVec2      DisplayWindowPadding;       // Window position are clamped to be visible within the display area by at least this amount. Only applies to regular windows.
     ImVec2      DisplaySafeAreaPadding;     // If you cannot see the edges of your screen (e.g. on a TV) increase the safe area padding. Apply to popups/tooltips as well regular windows. NB: Prefer configuring your TV sets correctly!