Fix PushFocusScopeID() + using shared stack. Renamed GetFocusScopeID() to GetFocusedFocusScope() - the two existing functions name are very error prone.

imgui.h

@@ -291,7 +291,10 @@ namespace ImGui
     // - Use child windows to begin into a self-contained independent scrolling/clipping regions within a host window. Child windows can embed their own child.
     // - For each independent axis of 'size': ==0.0f: use remaining host window size / >0.0f: fixed size / <0.0f: use remaining window size minus abs(size) / Each axis can use a different mode, e.g. ImVec2(0,400).
     // - BeginChild() returns false to indicate the window is collapsed or fully clipped, so you may early out and omit submitting anything to the window.
-    //   Always call a matching EndChild() for each BeginChild() call, regardless of its return value [as with Begin: this is due to legacy reason and inconsistent with most BeginXXX functions apart from the regular Begin() which behaves like BeginChild().]
+    //   Always call a matching EndChild() for each BeginChild() call, regardless of its return value.
+    //   [Important: due to legacy reason, this is inconsistent with most other functions such as BeginMenu/EndMenu,
+    //    BeginPopup/EndPopup, etc. where the EndXXX call should only be called if the corresponding BeginXXX function
+    //    returned true. Begin and BeginChild are the only odd ones out. Will be fixed in a future update.]
     IMGUI_API bool          BeginChild(const char* str_id, const ImVec2& size = ImVec2(0, 0), bool border = false, ImGuiWindowFlags flags = 0);
     IMGUI_API bool          BeginChild(ImGuiID id, const ImVec2& size = ImVec2(0, 0), bool border = false, ImGuiWindowFlags flags = 0);
     IMGUI_API void          EndChild();