Fix up and merge Wayland threaded event loop

There is a patch to move event processing to a separate thread, which fixes a deadlock when Qt Wayland is used together with components that do their own event processing (such as the GStreamer video sink).

However, the patch is no longer applicable to the recent updates in the code base and needs some modifications. There has been a discussion of how to make it compatible with the recent changes, which is about adding new event queues for frame updates, and no real consensus yet.

It would be really good to get this change in, so it should be investigated properly and we should try to find a solution for it.