WIP: backend: Initial backend crate with Room model

This patch introduces the new fractal-backend crate. The idea is to move all the data management and data logic there so the fractal-gtk will be only responsible of the UI part.

This first partch introduces a first implementation of the data model for the Room struct so we can store in filesystem using SQLite.

The implementation defines a new Trait called Model so we can change the data model backend in the future to use other memory or filesystem storage.

Future commits will introduce all data models and relations between them.