Add two new requests to the gtk_surface interface: set_modal and
unset_modal. The server will currently not do anything special with
input focus, and its up to the client to ignore events on the parent
surface.

This commit bumps the gtk_shell interface version to 2. By connecting to
a Wayland server with another gtk_shell interface version any features
depending on the gtk_shell protocol will not be available.

https://bugzilla.gnome.org/show_bug.cgi?id=745721