Adds itemsWrap:bool (default false) to ScrollableList and PageableList. If itemsWrap is true then scrolling past the last item wraps around to the first. Similarly, scrolling before the first item wraps around to the last.
Added abstract ExtentScrollBehavior of ScrollBehavior. Renamed fields called contentsExtents to contentExtent, containerExtents to containerExtent, contentSize to contentExtent, etc.
BoundedBehavior is now a subclass of ExtentScrollBehavior.
Added UnboundedBehavior subclass of ExtentScrollBehvaior; contentExtent and maxScrollOffset are double.INFINITY, minScrollExtent is double.NEGATIVE_INFINITY.
Changed the pageable_list.dart example: tapping on the toolbar changes
the scroll direction. This exposed some problems:
- Scrollable.syncFields() didn't update scrollDirection
- Viewport updated its RenderObject fields in the wrong order
- FixedHeightScrollable scrollDirection changes didn't update the scrollBehavior
There may be similar problems with VariableHeightList and ScrollableViewport.
I will fix those in a separate CL.
