Move Toolbox into Scene View

Unity now has [customizable overlays](https://docs.unity3d.com/6000.0/Documentation/Manual/overlays-custom.html) that we should use for assets that aren't in the asset lib, such as walls, ramps, rubbers, the trough, metal guides and lights.

At the same time, enable quick access to the asset library. Maybe even with links to direct filters like for spinners, targets, etc.