Add support for app-menu button in window decorations

We want to synchronize the button layouts of our server side
decorations and GTK+'s client side ones. However each currently
may contain buttons not supported by the other, which makes this
unnecessarily tricky.
So add support for a new "appmenu" button in the layout, to display
the fallback app menu in the decorations.

https://bugzilla.gnome.org/show_bug.cgi?id=730752