doubleclicks a .py, .pyw or .pyc file. It runs the file by invoking the
relevant interpreter (either the command line Python in a terminal window
or a Python.app for GUI-based scripts). Interpreter to use and the options
to pass are settable through preferences.

If PythonLauncher wasn't running it does its thing for one script and exits.
If it was manually started before a dialog is presented where the user
can set the options to use, etc.

To be done:
- option-drag/doubleclick should always open the interactive dialog
- Terminal-window isn't done yet
- Should be reimplemented in Python, but pyobjc isn't part of the core.
- Various menu entries should be disabled.