Blender Git Loki

Fix/cleanup RNA viewlayer API.

RNA's ViewLayer would present 'first level' of layer collection as a
list (collection property), when it is actually now only a single item,
same as the scene's master collection.

Note: did not try to update view_layer python tests, those are already
fully broken for quiet some time I guess (they still assume
view_layer.collections to be mutable e.g.)...