quartz: add a default application menu

quartz: add a default application menu

When running on quartz, it is no longer expected for applications to
provide their own application menu.  Instead, they should simply ensure
that they provide "app.about", "app.preferences" and "app.quit" actions
(which many apps are already doing).

A default menu will be shown that looks like the one presented by all
other Mac OS applications, containing menu items for the above actions,
as well as the typical "Hide app", "Hide Others and "Show All" items and
the "Services" submenu.

If an application does explicitly set an application menu (via
gtk_application_set_app_menu()) then it will be respected, as before.

https://bugzilla.gnome.org/show_bug.cgi?id=720552