#762 SVG Export: vertical text alignment

[Alexey Stukalov]

When exporting the graph to SVG, the node labels are not centered to the nodes (when "center" is specified). I believe that's because alignment-baseline="middle" attribute has to be added to the <text> element when writing SVG. The label positions become correct, when this attribute is manually added in the SVG file.</text>

I hadn't tested, if the other label placement options are exported correctly.