Fix all touches ending on any touch ending.

The drag behavior registers a touchend listener for each started touch; however,
a touchend event is dispatched to ALL listeners when any touch ends, not just
for the corresponding starting touch. The drag behavior must therefore detect
whenever the ending touch is the corresponding starting touch, and ignore other
ending touches.

This fixes the drag behavior during multitouch, as discussed in #1786.
Add d3.touch; fix #1786.

The drag behavior no longer crashes when the element being dragged is removed
from the DOM. In addition, the new d3.touch method extracts a single identified
touch from the current touch event, making it more efficient during multitouch.
The drag behavior now assigns touchmove and touchend listeners on the target
element of the touchstart event, rather than the window.
d3.touch defaults to changedTouches.

This way, it’s easier to tell whether the touch changed during the event. This
also fixes #1600 because the drag behavior now only dispatches a drag event on
elements that moved, even if multiple touches are active.
Remove workaround for WebKit bug 44083.

This was fixed in 2010, so I think it’s safe to remove the workaround now.
Listen to the touch target.

This ensures that if the touch target is removed from the DOM during a zoom
gesture, the zoom behavior continues to receive events; touch events, unlike
other events, are always dispatched to the target of the touchstart event rather than the window.
Merge branch 'fix-drag-touchend' into 3.4.4