Allow to use cloud projects on the desktop app

[4ian]

To do:

• [ ] Upload of resources when going from a local file project to a cloud project
• [ ] Export is now working only for local files, it must be adapted to handle any URL.
• [x] Preview should work
• [x] Rest of the editor should work

Other things:

• [ ] Add various checks for internet connectivity (and timeouts so that):
  • [ ] A warning is shown before launching a preview/export (also applicable to the web-app, and already done for the export).
  • [ ] Retry loading resources if they fail??

Should we improve the errors though when a resource can't be loaded? It's harder to rely on the assumption of a stable internet connection.