wayland: fix deprecation warnings

Replace the deprecated APIs with the modern variants (which
also give us full control on the versioning).
Also, note that we only support version 2 of wl_seat. Version
3 is for multiple pointer/keyboards for a client, but
we don't implement that yet. If a client requests version 3
of wl_seat or make a version 3 request, it will get a protocol error.

https://bugzilla.gnome.org/show_bug.cgi?id=707851