Improve Output View Feature

[pasbi]

The Output View Feature allows changing the view used for rendering during an animation.
Currently, this is implemented with a Boolean Property in every View object.
The thing is, it does not make sense to have more than one such view.
Hence, there is a sophisticated mechanism that disables that property for all other views, when it gets enabled in a particular view.

The problems are:

1. It's not good UX. The uniqueness is not obvious. User may wonder why the property in another view became false automatically.
2. It does not work in animations correctly. (I don't think this is a fundamental problem, the bug could be fixed, but because of (1), it's not worth it, presumably).