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
Actually implement opening the app menu

The last commit added support for the "appmenu" button in decorations,
but didn't actually implement it. Add a new MetaWindowMenuType parameter
to the show_window_menu () functions and use it to ask the compositor
to display the app menu when the new button is activated.

https://bugzilla.gnome.org/show_bug.cgi?id=730752
window: Add accessor method for gtk_theme_variant

This is useful information for UI associated with a window, so add
a corresponding method.

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