list-view: Get proper backing-uri for tree expanders

When using tree expanders, we have special cases for the backing uri. In GridView or ColumnView (without expanders), the backing uri is always the current view directory. With tree expanders, the backing uri is either the parent of the currently selected item, or the currently selected item (if it is an expanded directory). The same rules apply for multiple selection if there is a common parent.

This was the case in 42, and the code was dropped in 6708861e, and as an oversight wasn't added back in when tree expanders were reintroduced.

Fixes: #2888 (closed)