When upstream events/queries reach sinkpads of unlinked groups (i.e.
no longer linked to the upstream demuxer), this patch attempts to find
the linked group and forward it upstream of that group.

This is done by adding upstream event/query probes on new group sinkpads
and then:
* Checking if the pad is linked or not (has a peer or not)
* If there is a peer, just let the event/query follow through normally
* If there is no peer, we find a pad to which to proxy it and return
  GST_PROBE_HANDLED if it succeeded (allowing the event/query to be properly
  returned to the initial called)

Note that this is definitely not thread-safe for the time being

https://bugzilla.gnome.org/show_bug.cgi?id=606382