gtk: Handle seatless displays

gtk: Handle seatless displays

If you run weston with the headless backend, you get a Wayland
display with no seat, which is just fine by the protocol.

gdk_display_get_default_seat() returns NULL in this case. Various
widgets assume that we always have a seat with a keyboard and a
pointer, since that is what X guarantees. Make things survive
without that, so we can run the testsuite under a headless
Wayland compositor.