Make mouse-enter/exit notifications match mouse event propagation

`NOTIFICATION_MOUSE_ENTER` and `NOTIFICATION_MOUSE_EXIT` now includes
the areas of children control nodes if the mouse filters allow it.

In order to check if a Control node itself was entered/exited, the newly
introduced `NOTIFICATION_MOUSE_ENTER_SELF` and
`NOTIFICATION_MOUSE_EXIT_SELF` can be used.

Co-authored-by: Markus Sauermann <6299227+Sauermann@users.noreply.github.com>

scene/gui/control.h

@@ -368,6 +368,8 @@ public:
 		NOTIFICATION_SCROLL_BEGIN = 47,
 		NOTIFICATION_SCROLL_END = 48,
 		NOTIFICATION_LAYOUT_DIRECTION_CHANGED = 49,
+		NOTIFICATION_MOUSE_ENTER_SELF = 60,
+		NOTIFICATION_MOUSE_EXIT_SELF = 61,
 	};
 
 	// Editor plugin interoperability.