bpo-31460: Simplify the API of IDLE's Module Browser. (#3842)

bpo-31460: Simplify the API of IDLE's Module Browser. (#3842)

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.