Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.

We still need to tune the various parameters, but this patch is a start.

This changes Switch from being a subclass of the Toggleable widget to
being a standalone Component with a custom RenderObject. This is to
enable adding Switch-specific radial reaction animations in a
subsequent patch. The CustomPaint logic that Toggleable was using does
not provide a nice way for a class and its subclass to both participate
in deciding to repaint.