* Raises wl_seat supported version to 4 in both server and client
* Raises wl_keyboard supported version to 4 in wl_keyboard
* wl_pointer and wl_touch are still on version 3
* Raises minimum Wayland version to 1.6
If a resource is null, because e.g. the surface got already destroyed,
wayland will create an error while marshalling arguments causing
the connected client to in worst case abort.
Running real world applications (Xwayland) showed that it doesn't like
at all that we do a dispatch when we are going to flush. This caused
in a very reliable manner a "Connection closed" error in XWayland, taking
down the client and in return the (xwayland-enabled) server.
