gstpad: Add a new GST_PROBE_HANDLED return value for probes

gstpad: Add a new GST_PROBE_HANDLED return value for probes

In some cases, probes might want to handle the buffer/event/query
themselves and stop the data from travelling further downstream.

While this was somewhat possible with buffer/events and using
GST_PROBE_DROP, it was not applicable to queries, and would result
in the query failing.

With this new GST_PROBE_HANDLED value, the buffer/event/query will
be considered as successfully handled, will not be pushed further
and the appropriate return value (TRUE or GST_FLOW_OK) will be returned

This also allows probes to return a non-default GstFlowReturn when dealing
with buffer push. This can be done by setting the
GST_PAD_PROBE_INFO_FLOW_RETURN() field accordingly

https://bugzilla.gnome.org/show_bug.cgi?id=748643