ENH: #3284 don't let user edit some dicom fields

As of r22329 the name of the volume can be set via the table widget,
but editing the other fields makes no sense, so we disable it.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22330 3bd1e089-480b-0410-8dfb-8563597acbee