Support multiple projects

[jbilcke-hf]

Today everything goes to the data directory. We should allow managing multiple separate projects.

• Add a sidebar
• Refactor our app to have a list of models
• Save current project in $data/models/untitled/ dir
• Allow renaming a project (+ check it respect file convention)
• Allow creating a new project (+ check it respect file convention + name collision)
• Allow deleting a project (+ make sure we always keep an untitled project, recreate it if needed)

note: those changes are currently done in the multiprojects branch