wayland: Add GdkSeat implementation

wayland: Add GdkSeat implementation

GdkWaylandDeviceData conceptually gathers the data that belongs to
a seat, so it's been renamed (although the old typedef stays, plenty
of refactoring is due here...).

The methods in GdkSeatClass have also been implemented, the most
remarkable is ::grab, which ensures the grab is performed on all
the relevant "master" devices.

https://bugzilla.gnome.org/show_bug.cgi?id=759309