Add update dialog to gallery, open source part of #4626

This patch is a warmup to improving the visuals in the calculator demo.

Related to #4535

Fixes 3544

Turns out we have the same demo in the "Buttons" section now.
We don't need both.

* Pesto demo for the Flutter Gallery app.

* pesto.special.case

This demo burns battery continuously even when there's no change to the visual
appearance. This patch removes this demo to avoid giving the impression that
Flutter burns battery.

Related to #4120

*  Gallery demo start-time performance test

* move the material_gallery/demos.dart script

* rename all_demos.dart to all.dart

Use the route table to generate the list of screens to test in the smoke test.

The 'routes' table is a point of confusion with new developers. By
providing a 'home' argument that sets the '/' route, we can delay the
point at which we teach developers about 'routes' until the point where
they want to have a second route.

Using .of() to call mutating setters is not great practice since it ties

parts of the app together in ways that prevent reuse and composition.



Since people are now using the gallery app to copy from, let's use the

better practice of using callbacks.

Fixes #2353

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308