Switch dicom editor loading to be mitkIOUtil based

Replace the hitherto handcrafted way of loading dicom files via the dicom
editor to be in line with the method used by drag and drop or file open.
This includes

• Increasing the service ranking of the auto select reader to be the default
• Imlementing a directory loading prototype for dicom directories

Does not touch the RT specific code currently used for loading RTSTRUCT,
DOSE and PLAN via the dicom editor