Editable files

### Current behavior

- `<source-code>` files are read from filesystem
- they are read-only
- demos may be using the same files (e.g. running the displayed code)

### Desired behavior

- `<source-code>` files are optionally editable in the browser
- on change, they are persisted to disk
- the presenter may run the demo, then edit the file, then run again the demo on modified source