LibGUI: GDialog should close its nested event loop on window close.

Make GWindow::close() so we can override it in GDialog and quit from the
internal event loop when the window manager tells us to close ourselves.

The dialog will return GDialog::ExecCancel in these situations.

Libraries/LibGUI/GWindow.h

@@ -82,7 +82,7 @@ public:
 
     void show();
     void hide();
-    void close();
+    virtual void close();
     void move_to_front();
 
     void start_wm_resize();