Fix wayland event loop integration

[hwti]

Synchronize the Wayland event loop with WPEBackend-fdo (but without a wl_event_queue).

Now the events which are already pending in "check" are dispatched in "dispatch" instead of synchronously. This allows glib to handle priorities, which should be fairer.

This fixes at least :

• error handling : "check" failed repeatedly with "Wayland::Display: error in wayland read", but "dispatch" was not called so the source was not removed.

• deadlock when g_main_context_check doesn't check our source : It happens either when the set of file descriptor changes, or when an higher priority source is ready. "prepare" was called twice without "check", then wl_display_read_events blocked forever.

[blino]

@magomez @aperezdc : does it look ok?