Cleanup and fixing of various code related to hot spots and HTML labels.

Cleanup and fixing of various code related to hot spots and HTML labels.
In particular:
  HTML labels were not being initialized correctly when used with
graphs or edges
  HTML labels now inherent url info from the object and use that unless
a subpart overrides it.
  There is now more control over which parts of an edge are active. The
URL attribute still provides a default for all pieces, which can be
overridden. We now also give control over the edge and main label
separately, so now the edge, label, headlabel and taillabel can be
independently set.