Fix a deadlock in Futatabi when using MIDI devices.

Fix a deadlock in Futatabi when using MIDI devices.

If something in the UI wanted to update a light on the MIDI device
at the exact same time the operator pressed a button on said device,
we could get a deadlock. The problem was that MIDIDevice::handle_event()
would lock the MIDIDevice and then go to tell MIDIMapper that the
note was coming (which needs a lock on the MIDIMapper), while in the
other thread, MIDIMapper::refresh_lights() would first lock the MIDIMapper
and then call MIDIDevice::update_lights() with the new set of lights
(which nedes a lock on the MIDIDevice). This is a classic lock ordering
issue.

The solution is to simply make MIDIDevice::handle_event() not lock the
MIDIDevice for anything that calls MIDIMapper; it doesn't actually modify
any state in MIDIDevice, except when we hotplug new devices (and that never
calls MIDIMapper).