Fixed LabelSetImageIOTest failure
Fixed LabelSetImageIOTest failure

Reason was, that the layer index was not set properly if a new layer was added.

RabadanLab/MITKats,iwegner/MITK,RabadanLab/MITKats,NifTK/MITK,fmilano/mitk,iwegner/MITK,NifTK/MITK,iwegner/MITK,fmilano/mitk,iwegner/MITK,MITK/MITK,fmilano/mitk,RabadanLab/MITKats,MITK/MITK,iwegner/MITK,RabadanLab/MITKats,fmilano/mitk,fmilano/mitk,fmilano/mitk,MITK/MITK,NifTK/MITK,fmilano/mitk,iwegner/MITK,MITK/MITK,NifTK/MITK,RabadanLab/MITKats,NifTK/MITK,MITK/MITK,MITK/MITK,RabadanLab/MITKats,NifTK/MITK