[issue28097] IDLE: document all key bindings, add menu items for more.

[Terry J. Reedy]

Change by Terry J. Reedy <tjreedy at udel.edu>:


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versions: +Python 3.7, Python 3.8 -Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue28097>
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