This patch makes it easier to debug layerization issues.

Using ImageResource solves two problems:

1) Listeners can be notified synchronously when the sky.Image is already
   available. This change removes flash of 0x0 layout when moving an
   already-cached image around in the render tree.

2) In the future, when we support animated images, we can notify listeners
   multiple times whenever a new image is available.

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

This introduces the notion of event disposition and allows event
targets (widgets and render objects) to consume events that should not
be processed further. This is needed by the Switch component in the
Drawer in the stocks example. The Switch is embedded in a DrawerItem.
The Switch handles the gesture tap event to toggle its state and should
handle pointer events to allow swiping and draw its own radial
reaction. The DrawerItem also handles gesture taps to allow toggling
the switch value when tapping anywhere on the drawer and to draw its
own ink splash. When tapping on the switch, both the switch's render
object and the DrawerItem's listener are in the event dispatch path.
The Switch needs to signal in some fashion that it consumed the event
so the DrawerItem does not also try to toggle the switch's state.

Turns out there was a typo in lerpColor.

Fixes #277.

The idea is that AnimatedContainer is a drop-in replacement for Container that
provides implicit animations when its properties change.

R=mpcomplete@google.com