This change make it easier to track the position of the scroll view without
having to worry about the position object changing out from under the
controller.
* Improved defaults for scroll view primary-ness
* Vertical scroll views default to primary:true.
* Horizontal scroll views default to primary:false.
* If a scroll view is primary and it got a non-null inherited primary
scroll controller, it introduces a primary scroll controller inherited
with a value of null for its descendants.
ScrollController now multiplexes writes to all registered positions;
reads of position continue to assert that only one position is
registered. Reads still require a single position.
Also, move the creation of the ScrollPosition to ScrollController.
Finally, remove TestScrollable in favor of CustomScrollView, which is the
production version of this widget.
If you pass a ScrollController to a Scrollable2, you can use the controller to
read and write the scroll offset without having to find the Scrollable2State
object.
