[kwin_wayland] Add SeatInterface to server module

So far the Seat interface is provided together with pointer and
keyboard. As always touch is not yet implemented. The pointer interface
is still lacking the set cursor callback. Keyboard on the other hand is
complete.

Both Keyboard and Pointer have the concept of a focused surface and only
to the bound interface belonging to the same client as the focused
surface events are sent.

The change comes with a set of new auto tests also verifying the client
side which wasn't possible before as we couldn't fake events.