Revision 1211

(LegendTree.__init__): Add instance variable
        preventExpandCollapse and subscribe to expanding and collapsing
        events.
(LegendTree.OnItemExpandCollapse): New. Responds to expanding and
        collapsing events and will veto the event if it has been triggered
        by the user double clicking on a layer.
(LegendTree._OnItemActivated): Set preventExpandCollapse to indicate
        that an expanding/collapsing event should be vetoed.