.midi Export

[Commanderk3]

To export .midi this PR uses the library tones/midi.

[Commanderk3]

I am close to completing this feature 👍

[Commanderk3]

fixes:#4311

@pikurasa I think the PR is ready for review. Please check it. Next, I have to work on adding instruments to the MIDI file. After exporting, you can play the MIDI file in media players like Windows Media Player. Right now, it will only play on the piano.
Just like Lilypond the notes have to be in a Start block. Each Start blocks are considered as "tracks" in midi. If you import it in a DAW you can edit it as well.

[Commanderk3]

@pikurasa The midi export is finally done. Please review this PR.
@walterbender I made a tune and exported it to FL studio. It is working well. Although my windows player does not seem to understand the instrument. But it works fine with others.

final.mp4

For testing we can use : https://pianotify.com/import-midi-file

[pikurasa]

I tested https://github.com/sugarlabs/musicblocks/blob/master/examples/Holly-Jolly-Christmas.html, and the output seems to get off track after the third note.

This is the MIDI file generated while on your branch, HEAD is now at d343fd1f4 bug fix:
Christmas.zip

[Commanderk3]

@pikurasa, I have fixed the issue. I thought _midiData was filled instantly, but that is not the case. It takes some time to retrieve all the data. Like Lilypond and others, I have created an after-save function to generate a MIDI file when midi data is filled.

Here is the result :->

midi2.mp4