New graph attributes and attribute cleanup
New graph attributes and attribute cleanup

* new parameters *NODE-SHAPES* and *PREDEFINED-ARROW-SHAPES* avoid
  clutter and duplication in attribute lists

* removed duplicated :arrowhead entry

* new "splines" graph attribute

* new "tooltip" node and edge attribute

* "compound" graph attribute and "l{head,tail}" edge attributes: needed
  for edges between clusters and clusters/node

* "rankdir" graph attribute: added value "TB"

daveloyall/cl-dot