Implement live MIDI input

So this is my code to support live MIDI input. A new Label and a ComboBox is add to the UI, and a few codes for the live MIDI input. It's not too much code so it should be pretty easy to understand and merge them. Actually we can change the MIDI port on the fly but I didn't see the requirement to do that so I'll just do the same as reselecting the COM port, do the shutdownSequencer().