Rainbow mode expansion (feature)

[Rodrix33333]

Rainbow mode is amazing.

To make it even better I suggest:

1. be able to change the gradients (so it's just a hue animation of reds, for example. Or a rainbow of fire colors).

Maybe what we actually need to do is to add a new functionality to gradients: time and scale for the animation.

2. to make the speed time factor, change depending on how many notes are pressed.