Add "clean child duplicate descriptions" tool

[jzohrab]

Feature request pulled in from #104.

When attaching parent terms to terms, Lute currently applies the description of the term to the parent term if the parent term's description is blank. This is by design: I wrote it this way because when I'm reading, I want a description to be applied to the parent (root) form even if it's slightly incorrect, figuring that it's good enough for me.

Some users find this annoying, as the data is duplicated across different entries, causing some annoying data management hassles.

This feature request is for something like the following:

• show children with the identical description as their parents (Note: I'm not sure if this is useful or not)
• clear the description for children if it's the same as the parent

Note that this will only find identical descriptions, it won't find cases where things are even mostly the same.

These new functions could be added under the "Terms" menu as an initial spot. Perhaps there needs to be a longer-term "Tools" menu, but that can wait.