wayland-client: link all event-queues of each display into a list

We need access to all event-queues of a single wl_display object. For
instance during connection-errors, we need to be able to wake up all event
queues. Otherwise, they will be stuck waiting for incoming events.

The API user is responsible to keep a wl_display object around until all
event-queues that were created on it are destroyed.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>