Move control over the color of bones from bone groups to the bones
themselves. Instead of using bone groups (which are defined on the pose,
and thus owned by the object), the color is stored on:
- the bone (`struct Bone`, or RNA `armature.bones['bone_name'].color`)
- a possible override on the pose bone (`struct bPoseChannel`, or RNA
`ob.pose.bones['bone_name'].color`).
When the pose bone is set to its default color, the color is determined
by the armature bone. In armature edit mode, the armature bone colors
are always used, as then the pose data is unavailable.
Versioning code converts bone group colors to bone colors. If the
Armature has a single user, the group color is stored on the bones
directly. If it has multiple users, the group colors will be stored on
the pose bones instead.
The bone group color is not removed from DNA for forward compatibility,
that is, to avoid immediate dataloss when saving a 3.6 file with 4.0.
This is part of the replacement of bone groups & armature layers with
bone collections. See the design task at #108941.
Pull request: https://projects.blender.org/blender/blender/pulls/109976
042c5347f4