Send volume changes to controller

[wvengen]

Ad this moment, the volume sliders can not send midi changes. This is relevant for controllers with motorized faders and led-based (rotary or fader) controls.

Some controllers can also gradually change the volume, which doesn't work when volume changes sent by the controller are sent back to it. That's why this needs to be controlled as well.

This PR adds midi messages for all volume/pan/send midi events, and the ability to toggle echo for these messages. Tested on the Launchpad X (#309).

• Code-style consistency, use Event::SOURCE or SOURCE (and the enum constants).

[harryhaaren]

@wvengen, any opens around this? Anything I can do in 30 mins to move it along? (And my apologies that I haven't been working on this stuff at all in recent months - but that's real life for me atm : )

[wvengen]

Hi @harryhaaren thank you for your response! I'd love to get the PRs merged (even if it will take a while, also from my side).
If you could take a first look at the code and indicate the most important things to address, that would help. And also hear if you'd prefer the use of Event::SOURCE or SOURCE.

[Spengreb]

A quick test of this seems to work well. Will try a few different midi controllers and report back.

[Spengreb]

Everything besides the pan dial is working exactly as intended on the the two midi controllers i have (FCB1010 and Alesis V25).

Not 100% sure but perhaps pan isnt working because it isnt track specific? At least thats how it looks in the binds

[harryhaaren]

Cool cool - thanks folks. Anything todo here, or ready for merge?

[wvengen]

There's one codestyle question about whether to use SOURCE or Event::SOURCE. I think it's not important enough to block merging, but if a consistent codestyle is very important to you, you may want to provide guidance here. If you have limited time to spend, then I'd say: please go ahead and merge.