Fix expanding of baloo URLs in folders sidebar tree view

D8332 resulted in baloo URLs like `timeline:/yesterday` appearing in
Gwenview, however accessing those in the {nav Folders} sidebar led to
error dialogs. 86d75454 already solved this for directly clicking on
entries, here we apply the same fix for expanding those entries, i.e.
clicking on the small arrows in the tree view.

CCBUG: 387824