InputText: Add ImGuiInputTextFlags_CallbackEdit, selection helpers in ImGuiInputTextCallbackData(). Add simple InputText() callbacks demo.

imgui_internal.h

@@ -858,6 +858,7 @@ struct IMGUI_API ImGuiInputTextState
     float                   CursorAnim;             // timer for cursor blink, reset on every user action so the cursor reappears immediately
     bool                    CursorFollow;           // set when we want scrolling to follow the current cursor position (not always!)
     bool                    SelectedAllMouseLock;   // after a double-click to select all, we ignore further mouse drags to update selection
+    bool                    Edited;                 // edited this frame
     ImGuiInputTextFlags     UserFlags;              // Temporarily set while we call user's callback
     ImGuiInputTextCallback  UserCallback;           // "
     void*                   UserCallbackData;       // "