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

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.