[Gui] UserEditMode : add cross comments in case a new edit mode has to be added

src/Gui/Application.h

@@ -234,6 +234,9 @@ public:
     /** @name User edit mode */
     //@{
 protected:
+    // the below std::map is a translation of 'EditMode' enum in ViewProvider.h
+    // to add a new edit mode, it should first be added there
+    // this is only used for GUI user interaction (menu, toolbar, Python API)
     const std::map <int, std::string> userEditModes {
         {0, QT_TRANSLATE_NOOP("EditMode", "Default")},
         {1, QT_TRANSLATE_NOOP("EditMode", "Transform")},

src/Gui/ViewProvider.h

@@ -420,6 +420,9 @@ public:
      * you can handle most of the events in the viewer by yourself
      */
     //@{
+    // the below enum is reflected in 'userEditModes' std::map in Application.h
+    // so it is possible for the user to choose a default one through GUI
+    // if you add a mode here, consider to make it accessible there too
     enum EditMode {Default = 0,
                    Transform,
                    Cutting,