Add reader xml for DICOM reader.

Added two entries for the reader, which has two modes to load data. The
first mode loads a single DICOM file. The second mode looks at all files
in the same directory as the selected file and load them as planes in a
3D image.

Change-Id: I07e98507c59dae96fc5f6aa34e39354aa8a7091e
Merge topic '14975-add-dicom-reader' into next

83cc48ae Add reader xml for DICOM reader.