DVEventInterface :this abstract class is used by diagram view to manage is event action (mouse event).
For add new action to diagram view, we must to create subclass of DVEventInterface and give it to diagram view,
when diagram view get the new dvevent, they manage it, and delete it when action is finish.
git-svn-id: svn+ssh://svn.tuxfamily.org/svnroot/qet/qet/trunk@3329 bfdf4180-ca20-0410-9c96-a3a8aa849046
