xl.playlist is not threadsafe

[Dustin Spicuzza]

In general, exaile doesn't do a lot of thread safety stuff (though, there are some things that are explicitly threadsafe via the synchronized decorator), and most things run on the main GUI thread.

However, gstreamer runs it's own internal threads, and some of the audio event callbacks are called on different threads. In particular, the gapless playback mechanism calls queue.get_next() on a separate thread, which calls get_next() on the playlist, and the playlist may be accessed by the GUI at the same time.

Since everything seems to work at the moment, this doesn't seem like a particularly high priority thing. But, worth putting a bug out for it anyways.