Python bindings for Views

This allows Views to be implemented in a limited fashion via Python, and allows ImageView to be subclassed to view non-image plugs via a preprocessor which converts to images. I have deliberately exposed very little of the View API, as it's not locked down and one of the main motivators for this work was to be able to implement Views in python free from the API compatibility worries that would be present in C++. Hopefully having complete control over the preprocessor will be enough.