In ControlPanel, added the ActionListeners for the add/remove user/group
buttons. They not correctly perform their function, and the tree view in
the ACP is updated. There is a bug when removing users/groups where the
tree view disappears, but this can be fixed by setting the text in one of
the JTextAreas. In the ACP added a method to allow the ControlPanel to
update the tree view by calling it, or else the tree won't be updated.