ui: Implement "window doesn't respond" dialog on gnome-shell

This does allow us to use ClutterActors instead of lousy legacy tools
meant for shell scripting.

https://bugzilla.gnome.org/show_bug.cgi?id=762083

src/shell-wm-private.h

@@ -52,6 +52,9 @@ gboolean _shell_wm_filter_keybinding (ShellWM             *wm,
 
 void _shell_wm_confirm_display_change (ShellWM            *wm);
 
+MetaCloseDialog * _shell_wm_create_close_dialog (ShellWM     *wm,
+                                                 MetaWindow  *window);
+
 G_END_DECLS
 
 #endif /* __SHELL_WM_PRIVATE_H__ */