Desktop: Add left/right arrow keys as expand/collapse shortcuts for notebooks

[personalizedrefrigerator]

Summary

ArrowLeft and ArrowRight are listed as standard shortcuts in the ARIA Authoring Practices Guide. This pull request adds them as shortcuts to expand/collapse notebooks.

This was also requested on the forum and is related to #10795.

Testing plan

1. Click on the entry in the notebook list for a nested notebook and expand it.
2. Press ArrowLeft. Verify that the notebook becomes collapsed.
3. Press ArrowRight. Verify that the notebook becomes expanded.
4. Press ArrowRight again. Verify that the notebook remains expanded.
5. Press ArrowLeft twice. Verify that the notebook becomes collapsed and remains so.
6. Press Space. Verify that the notebook expands.

This has been tested successfully on Fedora 40 (Linux).