window: Do in-app notification for unstarring

When unstarring a file in the starred view the file disappear from the view immediately, with no visual clue how to get it back in case of accident.

This leads to confusion and is generally bad UX.

To fix it, implement an in-app notification for unstarring when doing it inside the starred view.

The commit has also some clean ups, and makes sure it removes all notifications before displaying another one.

Closes #239 (closed)