Add equality to ContentManager Content items.
Improve ContentManagerGUI to re-use existing nodes for the same Content item.
Fix issues with loading child nodes while opening to show a known child (link click).
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@2488 a9f3cfff-6c41-7446-b0ca-052bd761e6eb