bpo-31206: IDLE: Factor HighPage class from ConfigDialog (#3141)

This is the first half of a patch similar to the one for for bpo-31205.  It is being split into 2 PRs to avoid what happened with PR-3096 -- an incomprehensible diff that could not be cleanly backported to 3.6.  This half copies several methods of ConfigDialog and turns them into a new class.

Author: csabella