Applies media padding (e.g. iPhone X safe area insets) to the Material Design demos in the Gallery.

Covers the following demos:
* Buttons (via the TabbedComponentDemoScaffold change)
* Cards
* Expansion panels
* Grid list
* Icons
* Images (via the TabbedComponentDemoScaffold change)
* Page Selector
* Progress Indicator
* Scrollable tabs
* Selection controls (via the TabbedComponentDemoScaffold change)
* Snack bar
* Tabs
* Text fields
* Tooltips

Fixes #13594

* make @immutable const

* fix build

Aggressively apply the const lint.

* Add cupertino to gallery and add CupertinoButto

* Use single quotes

* Add disabled state

* Some review notes

* Make button animation more responsive and tweak timing

* Renamed things Cupertino

* Button with background, move cupertino demos, move material demos

* Move 2 level list too

* Refactor various demo route names

* Some review notes

* More reviews and add test

* Linter as

* Move private constant up

This prepares us for a CustomScrollView that takes slivers.

Uses SliverPadding to implementing paddding in ScrollView. Also, deploy
ScrollView in more places now that it implements padding.

Finally, remove loader_app.dart because it is not referenced.

This patch deploys ScrollView in a number of demos and manual tests.

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

See https://www.google.com/design/spec/style/writing.html#

Gallery Tweaks

* left -> leading (Removes an LTR bias)
* center -> title (Widget was actually centered)
* right -> actions (Removes an LTR bias, asymmetric with leading)

Fixes #2348

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

The example in the spec with multiple actions is an anti-example.

Fixes #1876