client: Queue display events on private queue and always dispatch

The wl_display events (error and delete_id) need to be handled even
if the default queue doesn't get dispatched for a while.  For example,
a busy EGL rendering loop hits wl_display.sync every eglSwapBuffers()
and we need to process the delete_id events to maintain the object ID
data structure.

As it is, that doesn't happen, but with this change we special case
wl_display events.  We put them on a custom, private queue and when
dispatching events, we always dispatch display_queue events first.
The wl_display proxy should still be the default_queue, so that objects
created from wl_display requests get assigned to that.