Keep the font size delta across settings reloads (#20230)

This is a trivial fix for an issue we get a somewhat outsized number of
complaints about.

When the user has adjusted the font size in any direction, we'll just
keep track of the magnitude and apply it every time the settings change.

Yes, this means that if you zoom once and then change your real font
size it's going to zoom even more.

That probably doesn't matter.

Refs #11522