Document icons

### Checklist

- [x] I added a descriptive title
- [x] I searched open requests and couldn't find a duplicate

### What is the idea?

Is it possible to specify the icons for `file_extensions` separately from the icon used for the shortcut? Normally, the shortcut would be the application icon, and the files would be a separate document icon. But I don't quite see how to do that? It seems like when I set `icon` it's used for both.

If not possible, it would be nice feature.

### Why is this needed?

Normal application conventions.

### What should happen?

Additional `document_icon` key(s) in the `.json` file to specify the icon(s) to use for the `file_extensions`.

### Additional Context

_No response_

Start by locating where menu item JSON definitions read the existing icon and file_extensions fields, then trace how icons are installed for each supported platform. Define the document_icon field behavior and verify that application shortcuts retain icon while associated files use the document icon.

Written by the indexing model from the issue text.