When the selected tab changes in a TabBar for which isScrollable:true, animate the selected tab to the center of the viewport, if that's possible.

Added an optional duration: to Scrollable.scrollTo(newOffset, duration: d) which animates the scroll to newOffset.

Refactored the Scrollable implementation a little to make the roles of the "toEnd" and "toOffset" animations clearer.

_handlePointerDown() now stops both animations.

Similarly, we only call settleScrollOffset() from the cancel gesture handlers if the animations aren't already running.