Keep position in stacking order for deleted windows

Workspace::addDeleted swaps the Client with the Deleted in the
stacking order. For Unmanaged windows the Deleted is appended
to the stacking order which is the same layer.

When the deleted is closed the window is removed from the stacking
order.

The result is that a deleted window is no longer raised above all
other clients.

REVIEW: 104519
BUG: 158262
FIXED-IN: 4.9.0