Support multiple applications

#### Reason/Context
At the moment Studio only supports one application, but often you design multiple applications that are interacting with each other. So introducing something like a "project" could be really useful.

#### Description
We can still do local savings to the browser, it just enables us to integrate something like Cupid and other tools 🙂

Start by reviewing how Studio currently models its single application and how local browser saves work. Clarify the project boundary and interactions with Cupid or other tools; done means multiple interacting applications can be designed while local saves remain supported.

Written by the indexing model from the issue text.