wayland: Add and implement set/unset_modal for the gtk_surface interface

Add set_modal ond unset_modal to the gtk_surface interface. When a
surface is modal, the compositor can treat it differently from non-modal
dialogs, for example attach it to the parent window if any. There is
currently no changes to input device focus; it is up to the client to
ignore events to the parent surface that is wanted.

This bumps the gtk_shell version to 2.

https://bugzilla.gnome.org/show_bug.cgi?id=745720