collectpads: implement flushing seek support

collectpads: implement flushing seek support

Implement common flushing seek logic in GstCollectPads. Add new
API so that elements can opt-in to using the new logic
(gst_collect_pads_src_event_default) and can extend it
(gst_collect_pads_set_flush_function) to flush any internal
state.

See https://bugzilla.gnome.org/show_bug.cgi?id=706779 and
https://bugzilla.gnome.org/show_bug.cgi?id=706441 for the
background discussion.

API: gst_collect_pads_set_flush_function()
API: gst_collect_pads_src_event_default()

https://bugzilla.gnome.org/show_bug.cgi?id=708416