The duplicate operatos need to be a bit more complicated in this case,
because they need to assign the new geometry node group to the active
modifier. This adds an operator that duplicates the node group from
the active modifier and assigns the result to the active modifier.

Note that I think we might need to implement these operators in C to
do manual managing of users, right now the new and duplicate operators
don't reduce the user count of the original. But that can be done in a
separate patch if it actually is necessary.