Spin-off "demo" app components as separate package.

[daadu]

Time to spin-off the components of demo app that could be reused by other package maintainer to build similar "demo" app.

Here are some pointers/questions about it:

• What should we call it? - demo_maker, demoize, gallerize, or something else (suggest in comment)
• @WieFel I suggest you be the primary maintainer of the project and repo.
• Some feature still needs to be implemented to make it like gallery app - having toggle for fullscreen/mobile view. Although this is non-blocking, could be implemented in future.
• The proposed package should also be submitted to fluttercommunity org.
• backdrop - should use the planned package for it's demo app.

This issue will only be open until separate repo for it is created.

[daadu]

@WieFel Go ahead with it man!

[WieFel]

@daadu will think about it and come back to it!

[daadu]

Issue further discussed at WieFel/gallerize#1

[WieFel]

@daadu now using https://github.com/WieFel/gallerize for demo app, instead of manual implementation!