Revision 1780

Forward the map's TITLE_CHANGED messages
(ViewPort): Derive from Conduit instead of Publisher
(ViewPort.Subscribe, ViewPort.Unsubscribe): Use the new base class
when calling the inherited versions
(ViewPort._subscribe_map, ViewPort._unsubscribe_map): New helper
methods to subscribe and unsubscribe map messages
(ViewPort.SetMap, ViewPort.Destroy): Use the new helper methods to
handle the map subscriptions
(ViewPort.Map, ViewPort.map_projection_changed)
(ViewPort.layer_projection_changed): Add or update doc-strings