Refactor GaplessPlayback class

[einsteinx2]

I've done some work on this but it still is 1000 lines long and quite complicated. Some of that complication is unavoidable due to the fact we're streaming instead of playing locally and we support gapless playback, but there should be room for improvement

[einsteinx2]

alecgorge in the Slack group posted his own gapless playback implementation with BASS which we can look at. At first glance it uses a similar approach.

[einsteinx2]

Bumping to future as it works and is already partially refactored