- [client] Fix a potential race when the client destroys an object after pending events
  for it have already been queued in the event queue
- [client] Expose `Display::protocol_error()` to get the details of a protocol error
  that occured.
- [client] Don't try to connect to `wayland-0` if `WAYLAND_DISPLAY` is not set, this is a
  bad legacy practice which can cause clients to spawn in the wrong environment.
- [client] `Display` can now be cloned and sent accross threads as nothing prevents it