Add support for MIDI keyboard and other devices as an input method

[YuriSizov]

Originally suggested here: TerryCavanagh/boscaceoil#46, TerryCavanagh/boscaceoil#101.

This is requested quite often! Godot has native support for MIDI inputs (at least in theory), but I don't have a MIDI device to properly test this. Could try with an emulator, but I'd rather be able to replicate a real scenario before I commit to this.

Similar considerations to #57 apply though. MIDI devices are not guaranteed to have a full set of keys to cover the whole range of Bosca notes. So we need to somehow relate the inputs to an octave or other sub-range. And drumkits are once again a potential source of problems.