Adds `Scaffold#endDrawer` property to supply a second drawer to a Scaffold.

Gallery and stocks:
* Add 20x20 notification icons.
* Remove unused 76x76 (3x scale) icon.

Other examples: update JSON to Xcode 8.2.1 defaults

Fixes warnings for missing/unused icon assets in Xcode 8.2.x

I took the logo_square from:
https://github.com/flutter/identity-assets/tree/master/assets/logo_square
Opened it in preview, gave it a white background
and then dropped it in the icon tool to create
these updated icons.

We may want to use offwhite like we did for Android
but this is sufficient for my demo today.

@hansmuller

We now use a different approach for ios projects where the developer controls
the Xcode project file. This patch removes the old ".generated" approach in
favor of the new approach.