Clean up View implementations
Currently there are four classes implementing F3\FLOW3\MVC\View\ViewInterface in the FLOW3 package: AbstractView, EmptyView, JsonView and NotFoundView.
Some of them don't implement the interface correctly (assign() should return an instance of $this to allow chaining).
We decided to move the assignment functionality back to the AbstractView.
Obviously the code should still only reference the interfaces.
Besides, the AbstractView needs a cleaned up as it contains a lot of unneeded methods and objects.