Add option to save project to script

Add an option for users to save the project as a python script. 

- `ratapi.Project` already has a `write_script` method which should be used for this.
- Add a "Save Project as Script" menu to the `MainWindowView`.
- Clicking this menu should open a file dialog an get the filepath for the script then the script file should be saved to the  given path.
- Add appropriate unit test.  