Streamlined 2D menus and improved built-in 2D assets in 2020.2

@suxiangting Prefab Wheel idea: after seeing the new Toolbars UI ( Draggable overlays, custom toolbars, new Tool systems ) some kind of prefab container seems like a natural fit for it. Not a wheel, just a few clickable buttons in the same style as the rest of the toolbars.