Disambiguate horizontal and vertical scrolling

We now have separate gestures for horizontal, vertical, and pan scrolling.

Don't crash when no Android device is attached

Refactors a bunch of Android-related things into an AndroidDevice singleton class.

Fix crash when a MixedViewport is initially offset at a non-zero position

add/remove/add smoketest
basic vertical behaviour
basic horizontal behaviour

We need to always remove the widget when you sync a non-visible
widget, even if we already have it, because otherwise we'll try to
sync it with null again later, which causes a crash.

Test in #938.

Improvements to demo game

...and not when we mount and dismount.

Turns out that when we dismount, it's too late -- we've already set
renderObject to null.

We also mark the mixed viewport as dirty when it is removed from its parent.
Without this, we try to reuse the child nodes in subsequent syncs, which is a disaster.

Instead, we pump a onScrollUpdate with the offset if there is an offset.

Add scroll gestures and use them in Scrollable

This patch replaces the scroll gestures used by Scrollable with ones detected
by the ScrollGestureDetector.

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)`.

Stop exporting framework.dart from basic.dart, since we now have widgets.dart exporting all of framework.dart.

Use GestureDetector in the framework and examples

Add a --verbose flag to sky_tool

Also clean up some formatting issues.

We're now using it at the widget layer for everything except scrolling and
flinging.

Add tap, show press, and long press gestures and use tap in IconButton

These gestures use the new gesture detection system.

Add --release flag

Add GestureArena

In the GestureArena, gestures will fight it out to see which is the real
ultimate gesture.

Add basic Grid widget and test RenderGrid

Allow parents of RenderObjects that have layout callbacks to depend on those objects' sizes.

Fix infinite loop

Previously, if you used a layout callback, you could not have a parent
that did parentUsesSize, or if you did, you had to be marked
sizedByParent. With this patch, we allow the parent to depend on your
layout, even if you modify your child list during your layout using a
layout callback, by checking that the parent is still actively being
laid out in this scenario.

Add PointerRouter

Only require iOS 7 for Fitness app

This patch is the first step towards implementing gestures. The pointer router
allows the gesture detectors to hook in at the end of the pointer event
propagation chain.

Related to #145