wayland: Use a GdkSeat as key for our internal hash table

wayland: Use a GdkSeat as key for our internal hash table

With the current implementation, we use a `wl_seat` as the key for our
internal has table where we store the Wayland shortcuts inhibitors.

There is however no technical reason for this, and we could use a
GdkSeat instead, which will ease the implementation of the GdkToplevel
shortcut inhibition API.