Added ability to delete-and-shift notes in a track
Added ability to insert notes into a track
Refactored the way SoundSystem.currentPattern works (no longer an object, but an index into the patterns array)
Added ability to play a song end-to-end
Refactored the way the WebAudio graphs are constructed and managed across patterns with different track counts.
Added SoundSystem.playSong() method and associated logic.
Refactored the editor navigation code. Added start of selection structure.