QComboBox: fix item activation via completer

Task-number: QTBUG-31146
Change-Id: I64291f397d80bf934152f63e629810540abf466e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>