Channel groups improvement

Note: this essentially combines several suggestions, I can split them into different queries if you want.

Channel groups in dope sheet and graph editor are very cool features, but they are almost not used because of a lot of difficulties. This request contains some suggestions that will help alleviate the situation.

Move to channel Group operator

There is no trivial way to add a channel to an existing group other than deleting it and recreating it. The simplest solution is an operator that will allow you to do this. It may look like a move to collection operator, except that there will be no hierarchies

Join Groups

А banal operator that combines the contents of two groups

Drag on Drop

Probably the most desired feature for the channel list. It can work similarly to a Tree View or an outliner. Just look at how they work and it will all make sense at once.

Color Tag

channel groups already have color indication, but it cannot be assigned manually to a channel group, it is set depending on the bone color.

Where to get the colors for coloring is a separate question.