Issue #16893: Replace help.txt with idle.html for Idle doc display.

The new idlelib/idle.html is copied from Doc/build/html/idle.html.
It looks better than help.txt and will better document Idle as released.
The tkinter html viewer that works for this file was written by Rose Roseman.
The new code is in idlelib/help.py, a new file for help menu classes.
The now unused EditorWindow.HelpDialog class and helt.txt file are deprecated.