Extension to execute code outside the Python shell window.

This adds the following commands (to the Edit menu, until there's a separate Python menu):

• Check module (Alt-F5) does a full syntax check of the current module. It also runs the tabnanny to catch any inconsistent tabs.

• Import module (F5) is equivalent to either import or reload of the current module. The window must have been saved previously. The module is added to sys.modules, and is also added to the __main__ namespace. Output goes to the shell window.

• Run module (Control-F5) does the same but executes the module's code in the __main__ namespace.