Add command Populate Descriptions from Dicom.xml

[tznind]

The dicom standard descriptions for tags are avialable and embeddable within our dll (we can zip them and store as content like we do in BadMedicine.dll). So we can offer the user a command that populates all descriptions in one go across all Catalogues with the corresponding fields (where existing fields are blank)

We should have an option for right clicking also a single field and setting the description