ENH: Added option to disable DICOM reference check
- Added checkbox in the DICOM references dialog to not check for references again. Selection is saved regardless of pressed button
- Added generic options group in Application Settings / DICOM where this setting can be changed
- Added optional constructor argument to DICOMLoadable for easy conversion from qSlicerDICOMLoadable (useful for debugging when the otherwise deprecated python DICOMLoadable class is used internally)