Simplifying bookmark property models.

git-svn-id: https://svn.code.sf.net/p/genoviz/code/trunk@14743 80d5a9c2-7d8c-4c9f-9369-4e03bca4d2bd
Removing unnecessary classes.

git-svn-id: https://svn.code.sf.net/p/genoviz/code/trunk@14744 80d5a9c2-7d8c-4c9f-9369-4e03bca4d2bd
Merge remote-tracking branch 'upstream'