Empty book icon for unmodified / empty notebooks

When I'm prototyping I end up with loads of notebooks are called "Untitled - " most of them are empty, but some might have something in them.

It would be good if the empty notebooks had a different icon (maybe just an outline version of the book), I could immediately see which ones to delete.

Start by tracing how notebook entries and their icons are rendered in the notebook list, then determine how empty or unmodified notebooks are identified. Done means empty notebooks display a distinct outline book icon while notebooks with content retain their existing icon, making them easy to distinguish.

Written by the indexing model from the issue text.