Moved ImGuiColumnsFlags erroneously forward declared in imgui.h + demo bit.

imgui_internal.h

@@ -91,6 +91,7 @@ struct ImGuiWindowSettings;         // Storage for window settings stored in .in
 // Use your programming IDE "Go to definition" facility on the names of the center columns to find the actual flags/enum lists.
 typedef int ImGuiLayoutType;            // -> enum ImGuiLayoutType_         // Enum: Horizontal or vertical
 typedef int ImGuiButtonFlags;           // -> enum ImGuiButtonFlags_        // Flags: for ButtonEx(), ButtonBehavior()
+typedef int ImGuiColumnsFlags;          // -> enum ImGuiColumnsFlags_       // Flags: BeginColumns()
 typedef int ImGuiDragFlags;             // -> enum ImGuiDragFlags_          // Flags: for DragBehavior()
 typedef int ImGuiItemFlags;             // -> enum ImGuiItemFlags_          // Flags: for PushItemFlag()
 typedef int ImGuiItemStatusFlags;       // -> enum ImGuiItemStatusFlags_    // Flags: for DC.LastItemStatusFlags