Instead of incorporating the margin into the button, introduce a ButtonBar
widget that supplies the proper spacing between the buttons. Also, make these
buttons more configurable via ButtonTheme so that dialogs can change the
minWidth and padding of the buttons as required by the spec.

Fixes #1843
Fixes #3184

The dartdoc will continue until morale improves.

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.