Enhanced nodes: text and val_to_text

OpenSCAD allows horizontal and vertical alignment of text objects. The current node_text and node_val_to_text don't implement horizontal alignment and miss out the most important possible value, baseline, for vertical alignment.
So I have edited them to include these.
Options are:
valign: top, center, baseline, bottom. The default is now baseline.
halign: left, center, right. Default is 'left'

Note that a minor bug in GraphSCAD 1.0.7 makes it difficult to see values in the valign combo box as it is drawn under the halign one. The currently selected value is shown opposite the word "valign", so the value just above it is invisible.