The EventQueue is a wrapper for wl_event_queue. The usage is for the
case that the connection is in a different thread.

The EventQueue needs to be added to the Registry before Registry::create
is invoked. This ensures that the Registry gets added to the EventQueue
and all objects created by the Registry (and further down the tree)
will be added to the same EventQueue.

The auto-tests which already used a wl_event_queue are transited to the
EventQueue class. That's also the reason why there is no dedicated
unit test: the new code is sufficiently tested by the existing tests.