Added layers and multiple cameras per scene
Also some changes to the renderers list
(it's more like World's ComponentSystem list)

I don't yet know which is better: to store everything (layers, cameras) in a
scene, or to use separate scenes for each layer, associate scenes with cameras
(rather than the other way around), etc.
Added mouselook+movement components
And use them for the world camera.
A few other changes to components.