app: Close all closable windows from quit()

There's no relation between a window being hidden from overview/taskbars
and a window not being closable - currently we effectively disable the
fallback quit action for any application with open transients, which
simply doesn't make sense.

Instead, only exclude windows for which the close action has been
explicitly disabled.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/217

src/shell-app.c

@@ -1199,7 +1199,7 @@ shell_app_request_quit (ShellApp   *app)
     {
       MetaWindow *win = iter->data;
 
-      if (meta_window_is_skip_taskbar (win))
+      if (!meta_window_can_close (win))
         continue;
 
       meta_window_delete (win, shell_global_get_current_time (shell_global_get ()));