Revision 1074

* Thuban/UI/mainwindow.py (MainWindow.delegated_messages): Also
delegate SelectedLayer.
(MainWindow.LayerUnjoinTable): Implement.
(_can_unjoin): New. Helper function for the sensitivity of the
layer/unjoin command.

* Thuban/Model/data.py (ShapefileStore.OrigShapeStore)
(DerivedShapeStore.OrigShapeStore): New. Return the original
shapestore. Used to figure out how to unjoin.
(DerivedShapeStore.Shapefile): Fix a typo.