Move Getting Started and Tutorial to GitHub pages

Along with some other docs.

Allow setting of initial value in the Input component.

Make it possible to start a sky app in an arbitrary relative path, rather than requiring the dart file to be in the current directory.

...which I accidentally changed in a previous commit.

Avoid infinite loop in syncChildren().

This was a typo in #1062.

Also, some minor code cleanup and expose (in the app dump) the
_disqualifiedFromEverAppearingAgain boolean for debugging purposes.

These files really belong on other libraries.

Introduce package:sky/animation.dart

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

This makes the sync code stop if it would have to rearrange the
RenderObjects. I'll make it handle the cross-RenderObject case, as well
as the insertion-sync case, in subsequent patches.

The editing directory just defined two widgets. We might as well fold them into
the main widgets library.

Document and bring sanity to BoxConstraints

Turns out many of the functions on BoxConstraints weren't used or had callers
that could easily be updated to other functions. I've added dartdoc to all the
public functions as well as renamed some functions that had similar names but
did different things.

working on generating docs as part of the build

revert PATH changes

fix typo

some dartdoc changes in files

Center and Align should expand by default

This patch makes Center and Align expand by default, which is usually what you
want. It also adds a ShrinkWrap option to let you shrink wrap in one or both
directions if that's really what you want to do.

Add Dismissable unit tests

Add coverage for the DismissDirection paramter.

These needed to be updated because we moved the widgets directory into the src
directory to hide it from dartdoc.

Handle Widget instances being moved as-is to different parts of the tree.

Setup mac target for Fitness app

Improvements to demo game

They're just as crazy in this system as they are in the web.

```
enum DismissDirection {
  vertical,
  horizontal,
  left,
  right,
  up,
  down
}
```

To only enable dismissing to the right create the `Dismissable` with `direction: DismissDirection.right`. By default direction is `DismissDirection.horizontal` (left or right).

Updated the card_collection "Swipe Away" demo with a drawer that can be used to select one of the three X axis dismiss directions. Currently the MixedViewport class doesn't support horizontal scrolling, so the demo doesn't support the X axis dismiss directions.

Demo game improvements

Events should bubble up the tree