Updates: new engine, mixer and master effects

• The single file is split into modules: state, audio, rack, roll, synth, mixer, arrange, app, ui
• New audio graph: every sound goes through a strip with volume, pan, reverb send and delay send
• Mixer panel with faders, level meters, mute and solo
• Master section: 3 band EQ, reverb with room size, drive, compressor
• FL Studio style layout, click a panel title to fold it
• The song is now a proper data model (tracks, patterns, drums). Songs saved by stages 1 to 3 still open