Add gtk_application_prefers_app_menu()

Add gtk_application_prefers_app_menu()

Applications can call this to determine if they should an app menu.
This will be %FALSE on desktop environments that do not have an
application menu like the one in gnome-shell.  It is %FALSE on Windows
and Mac OS.

Applications are completely free to totally ignore this API -- it is
only provided as a hint to help applications that may be interested in
supporting non-GNOME platforms with a more native 'look and feel'.

https://bugzilla.gnome.org/show_bug.cgi?id=722092