Store the 'expanded/collapsed' state of the bone collection tree view in
the DNA data of the bone collections themselves. This way the tree state
is restored when loading the file.
This commit also adds some code to the abstract tree view classes, for
supporting synchronisation of the extended/collapsed state between it
and external data. It follows the same approach as the handling of the
active element.
RNA wrappers have been added to make it possible for Python code to
expand/collapse parts of the tree.
Library overrides are supported for this property, so the
expanded/collapsed state of linked armatures can be locally saved. If
there is no override, the `is_expanded` property is still editable;
changes will not be saved to file in that case, though.
Pull Request: https://projects.blender.org/blender/blender/pulls/116940
660867fa00