GTK: Improvements to the GTK+ player

This patch contains a few improvements to the gtk-player:
* Uses unified play/pause button.
* Adds a Skip Previous/Next button.
* Volume control
* Takes multiple files/uri's as arguments.
* Switches to the next track (if any) when the current track
  reaches and end.

https://github.com/sdroege/gst-player/pull/1
GTK: Fix logic for setting the prev/next button insensitive at the end/beginning of the playlist
GTK: Turn checks that must not fail into assertions in prev/next logic