We might want to let folks customize the inactive color too, but customizing
the active color is a good place to start.

After this patch, if you try to use a widget that depends on being enclosed in
a material, you now get an assert and a debugPrint if you're not inside a
material.

Fixes #243

Instead of PointerInputEvent having a "type" field, we now have a
different class for each pointer type.

This has ripple effects throughout the system.

I also did code cleanup in affected files while I was there.

Have the widget support min/max arguments to make the widget easier to
use. Also, fix the dragging so it's actually where the knob is.

 - These controls now have proper radial reactions.
 - You can drag the switch.
 - The radio button animates properly.
 - There's a demo in the Material Gallery

Also, have the slider draw different in its disabled state.

The slider in this patch works, but it's missing animations and many subtle
touches.