We now have separate gestures for horizontal, vertical, and pan scrolling.
Add a way of having keys based on numeric types or DateTimes by having a ValueKey<T> class. Remove the redundant ways of declaring things, except for leaving one shorthand -- you can say `new Key(s)` instead of `new ValueKey<String>(s)`.
These gestures use the new gesture detection system.
Also includes changes to widget_tester sufficient to test scrolling. @abarth
