ViewLayer + Render Nodes

This idea is piggybacking on @txo 's recent, and good "Render Nodes" proposal, but as a simple way to begin to achieve many of the same goals.

My proposal here is to simply split the RenderLayers node into two separate nodes: a ViewLayer node, and a Render node.

Each Render node could store it's own render settings, thus allowing the user to render each view layer with a unique render engine and quality settings. A Render Animation command would execute renders of each un-muted Render node in the compositor.

I believe this would add some very helpful functionality, while also making layered rendering much easier to understand an manage.