Remove elements/listChildren and use elementsv2 for acts

Part of #361 (closed): when an element has no zone but is not a folder (Himanis act), the frontend displayed the non-folder child elements with zones instead. This made a call to elements/listChildren every time a non-folder element is displayed, even when the element has a zone.

This uses links in elementsv2 to find the element's direct non-folder children with zones. This allows the frontend to handle more than 20 surfaces on an act and gets updated along with the children tree for example via annotations. As this was the last remaining call to elements/listChildren, the action, mutations and state have been removed.