Option to Avoid Duplication of Node Subgroups When Appending

Currently , when appending a node group, all subgroups of that group are also appended as new groups. While this behavior is generally desirable, there are cases where it creates significant extra hassle if those subgroups already exist in the file being appended to.

It would be quite helpful if there were a toggleable option to check for existing node groups with the same name and contents as the subgroups, and change the references in the appended group to point to them, rather than appending the duplicates.