Add "Save as" functionality to create copies of scenes

It would be nice to be able to take a scene I already have, save a new version and modify it so the original stays untouched. Currently, users can export and import a spoke file, but it would be nice if there was an easier way.

Either a "Save as" button inside the project, or maybe a "Duplicate" button in the project thumbnail dropdown (see screenshot).

Start by examining the existing export and import flow for spoke files, then inspect the project thumbnail dropdown and the in-project controls shown in the screenshot. Define how a scene copy is created and where it appears in the project list; done means users can duplicate or save a scene as a new version while leaving the original untouched.

Written by the indexing model from the issue text.