ENH: Add focus stealing prevention option

(Hopefully) implements the fix described in #1727 and #2445, and may have some impact on #2640.

I've never contributed to Mint before, so let me know if there's a something I should read about how to contribute in case I did something dumb. (I didn't see anything about "contribute" on the Cinnamon homepage or this repo's README, but it's possible I missed it.)