[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.