Fix crash with splitting and non-splittable editors

We may not just add an editor to a view without removing it from the
original view.

Task-number: QTCREATORBUG-9843
Change-Id: I1041ceb1f063bb627b5f0bd16bc3b2973a4dc3b6
Reviewed-by: Eike Ziller <eike.ziller@digia.com>