Feature/now playing

[arfbtwn]

There were a number of problems with the NowPlaying extension making its behaviour, particularly with respect to GUI components, a bit confusing. In addition there were a number fo hack comments indicating quality issues. It appears to have suffered some bit-rot as functionality was added and the extension was expanded.

We address a number of things:

• remove the unused substitute audio display;
• make NowPlayingContents default home in NowPlayingInterface;
• remove all unnecessary graphical programming;
• remove the OnShown timeout hack;
• re-work going fullscreen and back.

Finally we prevent background pattern clearing in the Gdk.Window we ask GStreamer to draw on, this causes an XLib error on reparenting when the HeaderBar is in use in the lite interface and the only reason I can find for clearing the property is the original GStreamer C example code for the video overlay.

[arfbtwn]

Would be grand to test this patch out with the regular interface in master branch once that's semi-up-to-date. Couple of highly visible outstanding issues:

• [x] going full-screen results in the primary window being hidden from the window manager; #20
• [x] cover art in the track information display doesn't render first time unless you activate the source first;
• [x] switching active window causes art refresh - should only happen on widget resize;

Stretch:

• [ ] full screen controls aren't visible. (Pretty sure this is an issue on master too, can handle this as a bug later)

[arfbtwn]

20 should now be resolved in this branch

[arfbtwn]

It'll be a little shorter once all the debugging statements I added are removed before merging...