Add leaf editing functionality

[chfin]

It would be nice to have some functionality to delete and re-insert leaves. It could work like this:

• leaves can be deleted just like inner nodes
  • issue a warning / require confirmation so that leaves are not deleted by accident and forgotten about
• add a tiny + button between every two leaves for adding new nodes
  • either add new nodes to youngest common ancestor of it's neighbours
    • increases arity, may lead to nonsensical trees
  • or remove all of the tree that reaches over the new node
    • removes rules but doesn't introduce unwanted rules
    • tree might have to be redone anyway