Make it possible to nest bone collections. The data structure on the
armature is still a flat array. It is organised as follows:
- Sibling collections (i.e. ones with the same parent) are stored
sequentially in the array.
- Each bone collection keep track of the number of children, and the
index of the first child.
- Root collections (i.e. ones without parent) are stored as the first
elements in the array.
- The number of root collections is stored on the Armature.
This commit also contains the following:
- Replaced the flat UIList of bone collections with a tree view.
- Updated the M/Shift+M operators (move/assign to collection) to work
with hierarchical bone collections.
- Updated RNA interface to expose only root collections at
`armature.collections`. All collections are available on
`armature.collections.all`, and children at `bonecollection.children`.
- Library override support. Only new roots + their subtrees can be added
via overrides.
See https://projects.blender.org/blender/blender/issues/115934
Co-authored with @nathanvegdahl and @nrupsis.
Pull Request: https://projects.blender.org/blender/blender/pulls/115945
129fb2eab8