Blender Git Loki

Animation: move group colors switch to user preferences

Move the "Show Group Colors" toggle from a per-editor option to a single
user preference in the Animation preferences. The Grease Pencil
animation channel side panel allows picking a channel color; this now
shows a message when channel colors are disabled.

The old "Show Group Colors" toggle had to be set per editor, and was on
by default. This meant that disabling group colors would require an
action for every file, for every editor. It is very hard to select a
color that works both as bone color in the 3D Viewport (needs to be
bright there) as well as the channel list (needs to be dark there), most
animators turn channel list colors off.

Differential Revision: https://developer.blender.org/D9391