[3.6] bpo-31459: Rename IDLE's module browser from Class Browser to Module Browser. (GH-3704) (#3710)

The original module-level class and method browser became a module
browser, with the addition of module-level functions, years ago.
Nested classes and functions were added yesterday.  For back-
compatibility, the virtual event <<open-class-browser>>, which
appears on the Keys tab of the Settings dialog, is not changed.
Patch by Cheryl Sabella.
(cherry picked from commit cd99e79d)