Reorganized TreeCellRenderer code to make a better workaround for Swing's memory leaks in in JTree.
See JDK-6507038.
Why the folks at Oracle still haven't fixed it is beyond me...

git-svn-id: svn+ssh://svn.code.sf.net/p/pcgen/code/Trunk@23916 ef53c05b-3124-4d59-ae7d-fe3273f42be7