It is possible to query the OS for the connected MIDI controllers,
but the event messages' device field was not being used. This implements
controller index being sent in InputEventMIDI messages in the device
property, matching the index from OS.get_connected_midi_inputs().
Based on the work done by @ramdor.
Closesgodotengine/godot-proposals#7733
Co-authored-by: Richie <richie_github@grange-lane.co.uk>