New DragDropStage menu controls the wrong tabs.

When tearing off a `Dockable` and dropping it outside the layout to create a new `DragDropStage`, the menu created for the new `DragDropStage` controls `Dockable`s in the original `DockContainerLeaf`. It does nothing to affect the new `DockContainerLeaf` in the new `DragDropStage`.

To re-create
1. Open the Basis demo.
2. Drag and drop the `Workspace` tab anywhere outside of the `Stage`.
3. Select the menu in the new `DragDropStage` and change it from `LEFT` to `TOP`.
4. Observe:
1. The `Workspace` tabs remains on the left side of the dock container.
2. The `Bookmarks` and `Modifications` tabs move to the top of their dock container.

This might be an isolated issue in the Demo application. I don't experience the issue in the persistence demo, but I build the Dockables differently there.

Start with the Basis demo and reproduce the tear-off flow using the Workspace, Bookmarks, and Modifications Dockables. Compare how Dockables are built there with the persistence demo, where the issue is not observed, and trace the new DragDropStage and DockContainerLeaf created after the drop. Done means the new stage's menu changes its own leaf without moving tabs in the original container.

Written by the indexing model from the issue text.