Bugfix: if a MultipleCDockable was stored as child of another
MultipleCDockable, storing and loading the layout failed. The reason for
that was a feature in DockFrontend.addDockable, where the layout of any
new Dockable was updated if an old layout was stored in the frontend. In
the case of a MultipleCDockable there is an old layout stored, because
they are "missing" at the time when the layout-file is read. The
solution is to be more careful about applying old layouts when loading a
new layout at the same time.