Allow font choice

Thanks for this. I usually use LibreOffice for csv work, but it's having trouble today with some upgrade issue so I was looking for something lightweight.

It would be great to be able to change the font, ideally through preferences, but even a simple +/- to increase or decrease the size would be welcome. It's also a little odd (to me) that it uses a non-monospaced font.

The issue names no files, tests, or entry points. Start by locating where the CSV editor sets its current font and whether preferences already support display settings; done means providing a usable way to change the font or its size and avoiding the current non-monospaced default.

Written by the indexing model from the issue text.