Issue #15678: Fix menu customization for IDLE started from OS X

Issue #15678: Fix menu customization for IDLE started from OS X
command lines.  It was broken as a side effect of the changes to
pythonw.c in b79d276041a8 for #15307.  Since sys.executable no
longer includes 'Python.app' in the path name, test for a
framework build instead.  This should give the previous behavior
in nearly all cases.  Whether the previous behavior is sensible
is left as an issue for later releases.  IDLE.app behavior was
not affected as it does its own manipulation of sys.executable.