TreeViewDelegate: Fix non-observable modelIndex property getting stuck

Although not quite consistent, the ItemBranchIndicators item could randomly get stuck without any usable modelIndex being ever set. It was caused by the readonly property var modelIndex not emitting any change signal, which is turn was caused by treeView.index(row, column) expression being non-observable AND evaluating before a model is set.

This whole fix is quite bad, and I'd rather use an original QModelIndex instead having to reconstruct it from bits and manually subscribing to events which are potentially affecting it; but TableView could only set an index required property which an integer that seems to identical to the row property.