#111972 made sure copied sockets have unique identifiers, but it didn't
cover the case of copying a panel and its children.
- Added an optional `uid_gen` argument to item copy functions which
generates new unique identifiers for copied items. If not specified
the items will retain the original identifiers (e.g. when copying an
entire node tree).
- Removed the `copy_items` panel function from the API. Only used
internally and requires a uid generator now.
Pull Request: https://projects.blender.org/blender/blender/pulls/112074
fc321739f0