This implements seeking in the media player. It also makes things work with GStreamer 1.0. Hopefully it should still work with GStreamer 0.10, but I haven't been able to test that.
Patch applies against my python3 branch.
See git commit log for further explanation of the changes: https://github.com/skagedal/liferea/commits/mediaplayer
You might instead want to merge directly from git so as to keep log entries:
git pull https://github.com/skagedal/liferea.git mediaplayer