Explicitly destroy static Glib::RefPtr<>s on exit (fixes #3636)

rtgui/cursormanager.h

@@ -82,6 +82,7 @@ private:
 
 public:
     void init                         (Glib::RefPtr<Gdk::Window> mainWindow);
+    void cleanup                      ();
     static void setWidgetCursor       (Glib::RefPtr<Gdk::Window> window, CursorShape shape);
     static void setCursorOfMainWindow (Glib::RefPtr<Gdk::Window> window, CursorShape shape);
 };