Add color_mode VelocityRainbow

[stephen322]

Adds a color mode to show the velocity of notes.

There were a lot of places to touch to add a color mode so I can't be sure I handled everywhere correctly. In particular, I don't have a menulcd, nor do I use sequences, so they are untested and probably incomplete.

[onlaj]

Hello.
Thank you for your contribution.
Indeed, the menu on lcd and sequences seems to be broken, I will try to fix them.
Also, settings.xml needs to be updated too, to include new settings, I will take care of it.

[onlaj]

Since you don't have waveshare screen I will make changes related to menu myself. Everything else looks fine.