[3.6] bpo-31460: Simplify the API of IDLE's Module Browser. (GH-3842) (#3843)

[3.6] bpo-31460: Simplify the API of IDLE's Module Browser. (GH-3842) (#3843)

Passing a widget instead of an flist with a root widget opens the option of
creating a browser frame that is only part of a window. Passing a full file
name instead of pieces assumed to come from a .py file opens the possibility
of browsing python files that do not end in .py.
(cherry picked from commit d6bb65f)