Instead of having to manage the popup menu from your app's build
function, you now just call showPopupMenu() with the menu's position and
it takes care of everything for you.

This solves the problem that the popup menu was trying to mutate the
state of the navigator from within its own initState() function.

Also, remove the "route" argument to RouteBase.build() since it equals
"this" by definition...

Also, remove ModalOverlay, and instead put that logic in the navigator.

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.

These constants aren't useful outside these files and are cause noise in the
dartdoc.

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

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

This fixes some theoretical bugs whereby we were using hashCode to try
to get unique keys for objects, but really we wanted object identity.
It also lays the groundwork for a new GlobalKey concept.

I tried to keep the impact on the code minimal, which is why the "Key"
constructor is actually a factory that returns a StringKey. The code
has this class hierarchy:

```
   KeyBase
    |
   Key--------------+---------------+
    |               |               |
   StringKey    ObjectKey       UniqueKey
```

...where the constructors are Key and Key.stringify (StringKey),
Key.fromObjectIdentity (ObjectKey), and Key.unique (UniqueKey).

We could instead of factory methods use regular constructors with the
following hierarchy:

```
   KeyBase
    |
   LocalKey---------+---------------+
    |               |               |
   Key      ObjectIdentityKey   UniqueKey
```

...with constructors Key, Key.stringify, ObjectIdentityKey, and
UniqueKey, but I felt that that was maybe a more confusing hierarchy.
I don't have a strong opinion on this.