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

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

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.

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

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.

```
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.

These functions are now in sky:dart.

In my previous patch, I forgot to fill in the other diagonal entries in the
device transform matrix.

Everyone uses sky.view.scene now. This patch also cleans up the raw examples
and makes them follow a consistent pattern.

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

Very simple so far. This schedules an alarm to fire once a day, kicking
off a service that downloads a new app.skyx from a hardcoded URL. The
new skyx replaces the current one.

This CL deletes a bunch of unused IDL files and removes some dead code in the
engine.

Fixes #247.

We need the new version because of the TextPainter rename.

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)`.

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