[issue37530] IDLE: simplify, optimize, and clean up code context

[Terry J. Reedy]

Terry J. Reedy <tjreedy at udel.edu> added the comment:

PR 14030 for #17535 (line numbers) also has code for 1. I want to merge this first, and fix-up the other.

Changing context font at the same time as the text font, instead of up to 1 second later, eliminates a visual glitch and is a user-visible improvement.  Ditto for highlight changes, although the situation is more complicated due to text recoloring (a separate issue).

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37530>
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