Player improvements

This pull request includes player improvements:
* proper asynchronous seeking
* enable seeking while playing
* remove stop-button
* use playpause-button
* update button based on gstreamer state only

It addresses the comments from the earlier (media-service-demo) pull request: e.g. when playback reaches end of stream, player seeks to beginning and pauses.

I've tried to keep UI changes very simple, hoping not to hit the murrine theme warning. But I don't run ubuntu so can't test that one. If it still occurs on ubuntu, skipping the last commit (moving the button next to scale) might help .

Fixes #3