new plugin: harmonize group ratings

[anarcat]

This allows one to process a subset of a library, find and fix images that are in a group but don't share the same rating.

This is a draft. It still needs a usage, some better docs, and a thorough review. Rationale in:

https://discuss.pixls.us/t/batch-copying-ratings-in-group/48874

[wpferguson]

I finally have time to look at this. I'll test...