Fix Memory Leaks in the Graphical Editor

[philip-alldredge]

There are global references to the graphical editor once an editor is closed. This prevents the objects from being garbage collected.

• AgeDiagramBehavior - Post Selection Listener
• AgeDiagramBehavior - Part Listener

There appears to be some other interaction with Eclipse that is occurring that is causing the editor to be retained until a new editor, textual or graphical, is opened. This latter issue will be ignored for now as it is uncertain whether that is normal Eclipse behavior.