ENH: Add new DICOM export feature and scalar volume plugin

DICOM export dialog and tag editor widget are new widgets for DICOM export of all kinds of data in the new DICOMLib directory of the Scripted module folder. The existing DICOM plugin mechanism is used, but extended with an export function. Although the existing DICOMLoadable python object still works, C++ containers have been added to ensure C++ support of DICOM import/export. DICOMDataExchange.py was split into DICOMExportScalarVolume.py and DICOMExportScene.py.
See all details, including design considerations and usage information here:
http://www.slicer.org/slicerWiki/index.php/Documentation/Labs/DICOMExport

Closes #3163

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23810 3bd1e089-480b-0410-8dfb-8563597acbee