Previously, we were nulling out the ArenaEntry in MultiDragPointerState
too early, which was prevent MultiDragPointerState from rejecting the
gesture in `dispose` if we hadn't accepted by the time the pointer went
up. Now we properly reject the gesture, which causes the tap gesture to
win during `sweep` in the arena.

Also, add a bunch of docs and annotations based on information I learned
while studying this issue. Finally, rename a private member of tap
recognizer to a name that would have confused me less in my
investigation.

Fixes #1186