MENUS: Context Menus - grab right-click for graph links
rt-click menus currently overridden for all objects on graph except for links -
these currently have a context menu containing one item - "Configure".
Need a design for the new context menu for links, and need to implement it by
addign the config getter code (as seen in
src/exp/ptolemy/vergil/basic/ParameterizedNodeController) to the constructor of
the link controller, which is ptolemy.vergil.actor.LinkController