These files really belong on other libraries.

If there are no other gestures in the arena, we should kick off the scroll
gesture right away. This change pulled a refactoring of how we dispatch events
to Widgets. Now we dispatch events to Widgets interleaved with their associated
RenderObjects. (Previously we dispatched to all of the RenderObjects first.)

This patch is the first step towards implementing gestures. The pointer router
allows the gesture detectors to hook in at the end of the pointer event
propagation chain.

Related to #145