Add a new interface, gtk_shell, than can be used by gtk to
retrieve a surface extension called gtk_surface, which will be
used to communicate with mutter all the GTK extensions to EWMH
https://bugzilla.gnome.org/show_bug.cgi?id=707128
Add support for GTK application menus
To do so, we need to be able to set surface state before creating
the MetaWindow, so we introduce MetaWaylandSurfaceInitialState as
a staging area.
The gtk-shell-surface implementation would either write to the
initial state, or directly to the window.
At the same, implement set_title and set_class too, because it's
easy enough.
https://bugzilla.gnome.org/show_bug.cgi?id=707128
