Various and sundry code cleanup

- Add documentation for AnimationTiming.
- typo: defaules -> defaults.
- added type information to isWatching() signature.
- made Widget.toStringName() include more useful information.
- cleaned up StatefulComponent._sync(): more specific signature, change
  redundant if to else, remove redundant cast.
- change order of TransitionBase arguments for consistency.
- prevent TransitionBase from affecting the performance in its
  constructor when it didn't create it (but see #1103).
- remove TODO() from @mpcomplete... no, there is not currently a better
  way to inherit a constructor, unfortunately.

Almost done adding dartdoc to the rendering layer.

We now expose idiomatic setters for these properties. Eventually we'll remove
the setter functions.

We didn't mean to flag null ancestors for this debugging check.

Fixes #1140

Add some more docs to the rendering library

If the build stack got long before, it would get cropped.

Disentangle FontSize from Settings

Instead, just store the default font sizes in statics. These statics are
constants for now, but we'll probably make them configurable at some point.

Shrink wrap when constraints are infinite.

Remove init.dart

Rename scroll.dart to drag.dart

ParagraphBuilder should be able to build a paragraph

The gesture is now called "drag". The library name should match.

If it's a StatefulComponent, then it's ok to reuse it so long as it
hasn't been initialised.

If it's a regular Component or a TagNode, then it's always ok to reuse.

If it's a RenderObjectWrapper, then it's ok to reuse so long as it
doesn't have a renderObject.

To put it another way, this changes how we prevent the following
nonsensical situations from arising:

 - Sync two stateful StatefulComponents together
 - Sync two RenderObjectWrappers with RenderObjects together

When either of those cases happen, we just drop the old one on the
ground and use the new one unchanged.

This patch start down the road of implementing text layout and painting without
the DOM. We can construct a basic paragraph consisting of a single run of text
and we can get through layout without crashing.

Dismissable now only depends on GestureDetector.

Added a unit test that verifies that issue #1068 has been fixed. It's commented out for now.

Cleaned up VelocityTracker.cc et al a little.

We haven't used init.dart in several releases. We can safely remove this no-op
script now.

That way the fling engages in the same direction as the scroll. For example, if
you have a horizontal scroll nested inside a vertical scroll, the fling will
take place in the same direction as the scroll.

Don't register gesture detectors when Scrollable can't scroll

Previously, if you used a number of nested Blocks, they'd each try to register
drag gesture detectors even though they can't actually scroll. This CL teaches
Scrollable to watch for drag gestures only when it can actually scroll.

The version and update URL are stored in the app's manifest (sky.yaml).

Look in $ANDROID_HOME/platform-tools/adb too

These are now declared in dart:sky as part of ParagraphBuilder.

Remove "disqualified" concept

Allows a non-initialised stateful component to be used as a source of
settings more than once. Instead of asserting that it was only being
used once, we assert that once you are stateful you don't get used as a
bag of settings, which is the real thing we were trying to avoid.

A lot of code ends up using StatefulComponents as a source multiple
times in a row, and before this would fail.

Patch by Ian Hickson

mine_digger: Fix not being able to de-flag on Android

Added FlingGestureRecognizer and exposed it in the GestureDetector class. FlingGestureRecognizer is based on the Android/Chromium VelocityTracker class which computes a velocity vector for for a list of X,Y,Time tuples.

The Scrollable classes now use the FlingGestureRecognizer. Dismissable and Drawer still need to be updated

Only log the tag and the message from Android, so logs look like “ANDROID: I/sky     : Hello from Sky!”.

Only log the tag and the message from Android, so logs look like “ANDROID: I/sky     : Hello from Sky!”.

Move Getting Started and Tutorial to GitHub pages