WIP: Support songs with multiple linked audio

Fixes #913

This includes the functionality for supporting songs with multiple linked audio, but the related tests have just been skipped. Also there are still lots of temporary print() statements in the code for debugging purposes.

I've created this merge request to solicit any comments about the overall approach, while I fix up the tests.

Supporting multiple linked audio files really forces us down the route of using VLC's MediaListPlayer. To avoid trying to do complicated things with times of media items, I think it's better to use the events which VLC emits to capture when items change etc.

So I've coded up this approach and applied to all the different sorts of media items which openlp supports, which means there are fairly extensive changes. The use of events does mean that openlp doesn't need to do any timing of media items itself, it just uses vlc's timing. So for example the tick() function in mediacontroller.py can be removed.