Fix splash timeouts on crimson

Since the touch data is released on touch up _gdk_wayland_seat_get_last_implicit_grab_serial will return 0 serial most of the time for touch only devices. This breaks xdg activation as the compositor will reject tokens that have a 0 serial set via xdg_activation_token_v1_set_serial.

Avoid that by tracking touch serials on the seat itself too.

/cc @jonathon.hall I need to test this in crimson but that's what I'm using on Debian