Demo: Add IMGUI_DEMO_MARKER (currently unused, but usage by altering GImGuiDemoMarkerCallback) (#3689)

Various amends/renames. (Amend to fix a warning)

imgui.h

@@ -152,7 +152,7 @@ struct ImGuiContext;                // Dear ImGui context (opaque structure, unl
 struct ImGuiIO;                     // Main configuration and I/O between your application and ImGui
 struct ImGuiInputTextCallbackData;  // Shared state of InputText() when using custom ImGuiInputTextCallback (rare/advanced use)
 struct ImGuiListClipper;            // Helper to manually clip large list of items
-struct ImGuiOnceUponAFrame;         // Helper for running a block of code not more than once a frame, used by IMGUI_ONCE_UPON_A_FRAME macro
+struct ImGuiOnceUponAFrame;         // Helper for running a block of code not more than once a frame
 struct ImGuiPayload;                // User data payload for drag and drop operations
 struct ImGuiSizeCallbackData;       // Callback data when using SetNextWindowSizeConstraints() (rare/advanced use)
 struct ImGuiStorage;                // Helper for key->value storage