Previously, the Slider used a drag gesture recognizer to move the head
of the slider, but when the slider was in a vertical scroller, the
recognizer would wait until the user moved the pointer by enough pixels
to disambiguate between sliding the slider and scrolling the scroller.

That worked fine for actual drags, but the slider should also move when
the user taps the track. This patch introduces a tap recognizer to
handle that behavior.

To avoid the slider's drag and tap recognizers from competing with each
other in the arena, this patch introduces the notion of a
GestureArenaTeam, which lets several recognizers combine to form one
entry in the arena.  If that entry wins, the team picks the first of its
recognizers as the winner, avoiding latency.

Fixes #7454