Make player engine use GObject signals

[Mathias Brodala]

Since the player engine is a singleton now which is accessible from everywhere, moving from xl.event events to GObject signals seems appropriate.

In this context a "position-changed" signal or similar should be introduced. This signal would be emitted automatically at a specified interval or after a seek to notify about playback position changes. This could slightly compensate for Gstreamer lacking such a signal.

[Dustin Spicuzza (dustin-virtualroadside)]

Given the player is no longer a singleton, this may or may not make sense any more. Would certainly be quite a bit of work, plus we'd need to deal with threading problems.