make sure the vertext labels are always in the foreground

[elbosso]

currently the vertex nodes (ovals, ...) and their labels are added to the FX hierarchy for each vertex - therefore the hierarchy holds "circle, text, circle, text, -..." But FX paints shapes in this order, therefore circles or other shapes added later obscure labels added earlier. Therefore i propose to add all the shapes for the vertices first and in a second loop add all the labels for the vertices - changing the order in the hierarchy to "circle, circle, circle,...,text, text, text,..."