The style we use for callbacks in widgets is "onFoo". These classes were using
an order naming convention and just called their callbacks "callback".

These are now part of material.dart.

This state object has a public API.

In the vast majority of cases, folks should be interacting with the Widget
rather than its State.

Fixes #267

This also moves the logic that tracks what Widget is being watched into
GlobalKeyWatcher, and much simplifies Mimic based on this.

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.

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

We weren't telling the new mimicable that we wanted to start mimicing it.

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

Fixes #751 and #756

Also, make RenderStack support negative positions with clipping so that we can
expand objects that are partially offscreen.

This widget lets you mimic one of its children in an overlay. The overlay
starts out as the same size of the child and then grows to fill the overlay. In
the future, the mimic will start at the same visual position as the child.

Wrap widgets you want to reparent in a Mimicable widget and assign the
Mimicable widget a global key. Then, given the same global key to a Mimic
widget to make it appear elsewhere in the view hierarchy.