Some minor code fascism.
Some OscServer improvements.

* Get rid of notifier signal from OscServer. It's going to be much easier to
  strongly couple it to the mixer.
* Add restricted-range float handler helpers for setting properties of the mixer over OSC.
* /firemix/global_speed and /firemix/global_dimmer now work.
Enable global_speed warping of tick time.
Add OSC handlers for next_preset, previous_preset, toggle_pause, and start_transition.
Convert buffer_rgb to integers before writing. Some transitions and presets write floats which causes exceptions when rendering the data to the network stream.
Add preset_duration, transition_duration, and toggle_freeze OSC handlers.
Fix bug where mixer was using indices to set next preset instead of the name.
Use mixer methods for next/prev instead of manual indexing in GUI.
Minor mixer cleanups.
Add stub set_position to AudioEmitter.
Emit changed signals on every change to the playlist model.
Remove all manual updating of the playlist view and instead rely on signals from the playlist model to drive updates.
Add a load playlist OSC command.
Add a mixxx_osc_port command line argument.
Add a /mixxx/control/set message proxy.
But don't printline.
Add method for sending Mixxx control updates from within Firemix.
Give AudioEmitters a target position to prevent Mixxx sending updates that over-write Firemix's goal position.
Make the default audio emitter position the center of the scene.
At the end of every tick, send a Mixxx control update for all AudioEmitter target positions.