preparing for ktmidi v0.4

[atsushieno]

I was not planning to make the version bump to v0.4 until today (there have been a lot of changes for 0.3.19), but I should record some big changes that would impact compatibility with former library usages here.

The first entry here is on the file format change for Midi2Music serialization. #23

[atsushieno]

This is also applied: #20 openInputAsync -> openInput, openOutputAsync -> openOutput.

Besides, now we have MidiAccess.name as an abstract property (which used to be open) so that it is now mandatory to override it in the derived classes.