feat: Add hard delete skeleton and skeleton subtree

[afonsobspinto]

Created by codex, used to test locally MetaCell/CATMAID#1

Leaving it as a draft as it might be useful to use for the same purposes by the reviewer

Some notes:

• Technically speaking the endpoints seem to be work correctly
• In terms of behaviour the delete of subtree is deleting the wrong part of the tree (imo)
• We need to add the new delete icon button to the selected widget (so we can delete subtrees from regular nodes)