Retire app-menu

For https://gitlab.gnome.org/GNOME/Initiatives/issues/4,
the application menu is being retired, with it's contents
being moved into the in-window primary window.

When removing a menu, the shortcuts for it's actions (if defined
in the menu) are also removed. The "Quit" shortcut is now
defined in src/gcal-window.c.

Related to https://gitlab.gnome.org/GNOME/gnome-calendar/issues/319

src/gcal-window.c

@@ -1570,6 +1570,7 @@ gcal_window_init (GcalWindow *self)
   gcal_window_add_accelerator (app, "win.change-view(2)",    "<Ctrl>2");
   gcal_window_add_accelerator (app, "win.change-view(3)",    "<Ctrl>3");
   gcal_window_add_accelerator (app, "win.toggle-search-bar", "<Ctrl>f");
+  gcal_window_add_accelerator (app, "app.quit", "<Ctrl>q");
 }
 
 /**