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Dependency of #33396. 
1. Base the initial font sizes on the on the configured editor font size. Note that editor font size defaults to 10 while the current base size is 12. I did not find any explicit discussion of this in the original issue, #16893, or the first followup, #25198. Perhaps Mark Roseman just liked the result, or perhaps it made the relative sizing easier.
2. Change font size with Control-Mousewheel. Manually tested code for doing this on Windows, Linux, and MacOS is in turtledemo.__main__. The three systems are wildly different. What we do here should allow easily doing same for other text windows.