- make undo and clear annotations work

...button/modules/whiteboard/WhiteboardCanvasDisplayModel.as

...rg/bigbluebutton/modules/whiteboard/business/DrawProxy.as

...gbluebutton/modules/whiteboard/events/WhiteboardUpdate.as

...uebutton/modules/whiteboard/managers/WhiteboardManager.as

...luebutton/modules/whiteboard/maps/WhiteboardEventMap.mxml

...igbluebutton/modules/whiteboard/models/WhiteboardModel.as