TreeView: fixed disappearing item
TreeView: fixed disappearing item

The problem occurs when a child object is no longer claimed by its
former parent. The child tree item is not added back to Document root
even if it is the only instance left, which resulting the child object
disappearing entirely from the tree view.

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master