Have a message in the menu (or a dialog) if no snippets are found

[ianhi]

If a user clicks on the snippets menu and there are no snippets perhaps a dialog could be raised that tells the user:

1. What folder the extension is looking in
2. That they should add .py files there
3. That they will then need to refresh their browser

I think point 3 is particularly important as this may not be obvious