Developer: Document Raven Widget Development

[JoshStrobl]

This is a pretty substantial task but the premise is to provide a clear step-by-step guide to developing a Raven widget. This should include:

• Documentation on the APIs themselves
• Documentation resources related to C and Vala
• Clonable templates for folks to build on top of, with documentation on our settings UI system
• Testing widgets, recommendations for backing up and restoring configuration