Add Multi-touch Support to Colored Circles Demo

Integrate multi-touch event handlers into the colored circles demo, for running on tablet devices such as the iPad. The semantics should be the same as the current mouse interactions.

As an example interaction, when the user taps all 5 fingers down on the device, 5 circles should be created under the fingertips. When the user places all 5 fingers back on those circles and starts moving, all 5 circles should follow the motions of the fingers simultaneously.