FBV support

[radiac]

Fastview is built around CBVs, but that can get in the way of putting together a simple view.

FBV support probably won't look entirely like a normal FBV, as some of the view logic makes sense as a CBV - but it should be possible to just write a simple view in a function.

[radiac]

Currently we're trying to define the view outside the class and assign it into the viewgroup - the intention was to implement native FBVs, but that's impractical, and doesn't make sense in the context of the viewgroup class.

Instead, define it as a method on the viewgroup. Behind the scenes we can wrap it in a staticmethod and turn it into a View class - just make sure to call it with the viewgroup as the first argument