Add a user-interface that makes it easy for people to change the reading of a character

This comes from #211.

The easier and probably more useful part is adding a user interface for changing the reading of a character. I think something is partially there in the preferences for left/right paren, although probably not implemented. That could be generalized in the underlying preference file.

Harder would be to allow users to change rules. I did an experiment in MathPlayer years ago that allowed users to make some changes to the rules. Might be worth looking into what I did to see if it makes sense to do to MathCAT.

Start by reading issue #211 and examining the existing preferences for left/right parentheses to determine what is already implemented and how preferences are stored. Review the referenced MathPlayer experiment for context, then clarify whether rule editing is in scope; done means a defined UI for changing a character’s reading works through the preference mechanism.

Written by the indexing model from the issue text.