Layered views

We may want to display some content on some separate layers, overlaying each other. One way to achieve this is to make the Views support multiple layers, and then draw then one by one. A Layer might be a separate class, so that the View becomes a container for a few Layers. By default, a View might construct with a single Layer already added (it makes no sense with no layers at all), and adding Drawable items directly to the View would in fact add them to that default Layer.