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.