Add EditorModel::openDocumentCount and use it.

As is, EditorModel::rowCount() could also be used, but this will most
probably change later. So we simply add a method that is independent
from all the "model" methods.

Change-Id: I39a2338d765ed6a269bf8977b816a5eda371afdf
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>