Added plug labels for hovered nodes.

- Added setLabelVisible/getLabelVisible to StandardNodule. Labels are oriented using NodeGadget::noduleTangent(), but are pushed away from the vertical to make them more readable.
- Added setLabelsVisibleOnHover()/getLabelsVisibleOnHover() to StandardNodeGadget. This shows the Nodule labels only when the node is hovered.
- Increased spacing between Shader nodules to make the text more legible.

This works, but because Gadgets don't receive enter/leave events during drags, we still need a mechanism for showing the labels when the node is being considered as a drag target.