Add midi beat event

[wvengen]

Implements #124.
Depends on PR #311, for this PR only the last commit here is relevant.

See also #124 (comment) for usage.

• Fix event queueing

[wvengen]

Thanks for your feedback and explanation! I'll change the ringbuffer issue you mentioned, will try to do so this year still.

[harryhaaren]

Hi @wvengen, reviewed these changes, they look good to me but I have not tested them. Any last comments, or are you OK with me clicking the big green MERGE?

[wvengen]

It's been a while, and I don't remember how thoroughly I tested it. Did some tests now (~10m), with the midi beat event and a midi monitor, some playing around, and midi input. All seems to work fine. I would be confident pressing MERGE (though for a production release, I would expect that someone else than me would also test it, but that could also be users and releasing another version if unexpected bugs come up; the code is straightforward enough that I don't really expect this though).
Thanks for keeping up maintenance on this project now and then :)

[wvengen]

Ah, actually there is one open question in PR #311 (note that this PR depends on it).
If you are happy with dataList, please go ahead. If not, then we should not yet merge it.