Move valuable sample code to relevant packages or framework

[guidezpl]

In my view, these are the most valuable demos:

• TwoPane → dual_screen cc @andreidiaconu

  • 

• Material/Cupertino demos: they should be nearly all already covered by 📚 cupertino library tracker flutter#102812, however, one may want to move individual gallery demos to a sample in the framework for the relevant API. cc @gspencergoog

[stuartmorgan]

• Motion → animations cc @stuartmorgan

@goderbauer is the primary owner of animations.

[goderbauer]

@guidezpl Could you link to the motion sample code that you have in mind?

[guidezpl]

They're in https://github.com/flutter/gallery/tree/main/lib/demos/reference, each starting with motion_

[goderbauer]

Those examples already exist in package:animations, see https://github.com/flutter/packages/tree/main/packages/animations/example/lib. I suspect, that they were actually copied from that package (with minor modifications like adding localizations) to the gallery 😄. I think we can remove these from the list of potential candidate for a move.

[guidezpl]

Indeed, with slightly modified names and a combined axis demo. For posterity, the demo looked as follows