Move sequencer related logic to sequencer namespace

[igorski]

Sequencing works via three actors:

Sequencer (static, collects events to playback at given time) AudioEngine (static, caches properties of the sequencer - time as samples -) SequencerController (constructor, directly modifies AudioEngine properties)

It is likely that SequencerController has a constructor for SWIG purposes when exposing it to Java (should still be possible to be exposed as a static class).

Move all cached properties out of the AudioEngine and into the sequencer to declutter the engine.