transport: Use the highest ranked, working decoder/encoder for VP8/H264 from the registry

This allows us to use existing codecs on the user's system, especially it
allows to use hardware codecs and removes a dependency on openh264.


Untested for now except for compilation, depends on
https://github.com/EricssonResearch/openwebrtc/pull/22
https://github.com/EricssonResearch/openwebrtc-gst-plugins/pull/3