Fix deserialized layout document close.

[pkindruk]

Recently I mentioned that LayoutElement._root doesn't have correct value after deserialization. When we deserialize layout from xml enclosing element is not finished with yet and therefore doesn't have correct parent yet.
This state prevents LayoutRoot.ElementRemoved from being called on close, which leads to skipped call to DockingManager.CollectLayoutItemsDeleted.

Also I mentioned that documents closed via deletion from DockingManager.DocumentsSource did not have their Content property set to null. Though we do it for AnchorablesSource and in ExecuteCloseCommand, so I added it.