First implementation of midi input

Hi,

I used frescobaldi 1.x in combination with rumor as static midi input for quite some time. Even as you started 2.x I stayed at 1.x since the new version dropped the rumor interface.

Now, for various reasons I switched to 2.x, but still the midi input was missing (even though long time announced).

So here is my attempt for a first implementation. There's still a lot missing, but since I am not an advanced python programmer, all this takes quite some time for me.

Greetings,
mamoch