Desktop: Fixes #10067: Fix dragging notebooks to the toplevel notebook

[personalizedrefrigerator]

Summary

Fixes #10067.

Testing plan

This pull request includes automated tests. However, it has also been tested manually by:

1. Dragging a notebook to the trash.
   • Verifying that it is indeed trashed.
2. Dragging the same notebook to the "NOTEBOOKS" header
   • Verifying that the notebook is moved out of the trash.
3. Drag the notebook to another notebook.
   • Verify that it is made a subnotebook of the other.
4. Drag the notebook back to the "NOTEBOOKS" header.
5. Verify that it is converted to a toplevel notebook.

This has been tested successfully on Ubuntu 23.10.