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: #117414
Rename `BoneCollection::is_visible_effectively()` to
`is_visible_with_ancestors()`. Soon a "solo" flag will be introduced,
and the effective visibility of the bone collection will depend on that
too. This particular function doesn't take that into account, though,
and thus needs a rename.
Note that this does NOT rename the RNA property
`is_visible_effectively`. That will be updated when the "solo" flag is
introduced to also take that into account.
No functional changes.
Pull Request: #117414
Sybren A. Stüvel