Implement Onvif Streaming specification

This brings in patches from !10 (closed) and #14 (comment 71220) , adds some modifications on top, then implements and tests the rest of the ONVIF streaming specification.

The test suite implements an example source, which lets us test and demonstrate the expected behaviour upstream elements should follow.

This is marked as WIP because this depends on a bunch of patches I proposed today, slightly extending GStreamer's generic seeking capabilities. Once demuxers / parsers implement these new mechanisms, we will have pretty decent out of the box support for ONVIF on the server side, which is nice :) The client side will require some attention to be functional, but I didn't identify anything truly involved yet.

CC @branko