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.