Don't divide by zero

If there aren't any items, there's no point in computing which items to show.

Add `Row` and `Column` widgets so you don't have to use Flex.

Remove floating-point errors from ScrollableWidgetList

We now compute which items to show using integers instead of floating point,
avoiding rounding errors.

Minor code cleanup

I updated everything in widgets/, but didn't update any examples.

New demo game

Minor cleanup in sky_tool

...rather than the stack trace at the time the RenderObjectWrapper updates the RenderObject.

Also, hide some of the debug-only fields behind "debug" prefixes and assert()s.

Better exception handling for rendering library.

Fix sky_tool listen on linux.

- Catch exceptions closer to the source.
- Factor out exception printing code.
- Have widget library hand the rendering library some context when syncing RenderObjectWrappers to aid with debugging.
- Fix a bug in flex.dart whereby _overflow was compared when null.

Mimic should track Mimicable more completely

We now have Mimic tracking Mimicable through tree structure changes and while
moving around the screen.

Fixes #751 and #756

Fix typography to use opacity instead of color value

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.

Add GlobalKey.registerAddListener

This lets clients listen for when a widget with a given global key goes through
a sync operation. We'll need this for mimic to track its mimicable when it
moves around the tree.

Mostly formating and removing arguments with default values.

DrawerItem, DrawerHeader should take a child instead of children

These widgets were designed in CSS where you need to specify a layout model for
your children. This patch updates them to the modern style of just taking a
unique child.

Fixes #755

Reimplement 'stretch' for flexible items correctly.

Cleanup how we manage scrollOffset in Scrollable

 - Introduce _setScrollOffset as a backend for the animations so that scrollTo
   can stop animations.

 - Create a single function that stops both kinds of scroll animations.

 - Refactor how we update the bounds for bounded scroll behaviors so that we
   update the bounds and compute the new scroll offset at the same time.

Fixes #698 to actually work.
Also, adds some debugging aids around Flex.
And a test to check this fix.