Changed ctkDICOMDataset to use DcmItem instead of DcmDataset

This commit contains only the necessary changes to enable an insert
of DcmItem into the database. The class should also be renamed to reflect
the change.
Fix FORTIFY_SOURCE warnings from gcc in Debug mode
Renamed ctkDICOMDataset to ctkDICOMItem

A fallback header and typedef were added to support old code
Renamed ctkDICOMDataset to ctkDICOMItem
Merge branch '255-replace-dcmdataset-with-dcmitem'

Closes #255

Conflicts:
	Libs/DICOM/Widgets/Testing/Cpp/CMakeLists.txt