Proposal Document for web&desktop

**This proposal is to add support for full screen flutter hosts like web and desktop . It is open for discussion and feedback. Let us know your ideas in comments.**

## List of Objectives
1. re-use "backdrop core" with different layouts/form other than `Scaffold`
2. **adapt** backdrop to larger screen sizes by making it **responsive** as defined in MDG
2. proper **physical keyboard and mouse** input support
3. add support for **keyboard shortcuts**
4. proper **tutorial guide** for usage with full screen hosts

## TODO
- [ ] #111 Abstract out "backdrop core" from `BackdropScaffold`
- [ ] #75 Backdrop with "horizontal layout"
- [ ] #17 Backdrop with `BackdropNavigationRail`

`TODO: add more TODO tasks once all objectives are clear and finalised`

## Quick Links
- Milestone: [web&desktop](https://github.com/fluttercommunity/backdrop/milestone/2)
- Project: [web&desktop Dev](https://github.com/fluttercommunity/backdrop/projects/3)