MIDI In triggers play button

[Saga Musix]

Modplug can respond to play/stop/continue commands from a midi device already, but this only works if your device supports sending special messages. So it would be good to have an option like "Play module on MIDI In" which starts playing the module from the cursor position as soon as MIDI data arrives (given that the module is not already playing).

Implemented in OpenMPT 1.17.02.53