Add the 'solo' flag to bone collections, effectively adding another
layer of visibility controls.
If there is _any_ bone collection with this flag enabled, only this
collection (and others with this flag enabled) will be visible.
In RNA, the following properties are exposed:
- `bone_collection.is_solo`: writable property to manage the solo flag.
- `armature.is_solo_active`: read-only property that is `True` when any
bone collection has `is_solo = True`.
The RNA property `bone_collection.is_visible_effectively` now also takes
the solo flag into account.
Pull Request: https://projects.blender.org/blender/blender/pulls/117414
a7f41fc938