Groups in approved color picker

[bjarnef]

Sometimes we have projects where several colors are added to approved color picker.
A few features that would be nice.

• Pin the color selector, so one doesn't have to scroll up to add more colors
• Allow groups similar to Block Grid. Sometimes it would be nice to group colors, e.g. in a multi-site solution, but as duplicate colors are not allowed, e.g. white, black or grey could be added at root (no group) or under a Shared group.