Tune up start-directory and last-directory behaviour, particularly for
Chirp on Windows.  This change is relatively benign for the user's
perspective.

start_dir
    o fix win32 file dialog start-directory issue in platform.py
      to support the open of correct or selected directory
      when requested.
    o add start_dir optional parameter to do_open() and do_import() methods.
      Include logic to pass start_dir requests to gui_open_file() in platform .

last_dir
    o capture last_dir information when files opened in do_open() and do_import methods.
    o pass last_dir as start_dir parameter in call to do_open()
      from ChirpMain's "menu helper" mh() method.