Also, fill out other documentation for these widgets.

Fixes #967

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

To make it easier to avoid pushing twice in one frame, provide a
transaction mechanism for the navigator.

The TimePicker can now actually pick a time. However, it doesn't understand AM
or PM and there's lots of visual polish missing.

You can't actually pick a time with the time picker yet, but this patch adds
most of the plumbing.