ENH: Add support for DCMTK private dictionary

See issue 4013 and related discussion here:

http://slicer-devel.65872.n3.nabble.com/Enabling-support-for-DCMTK-private-dictionary-td4034305.html

This change implements the following changes:

(1) install private.dic of DCMTK in a runtime location

(2) update launcher setting to set DCMDICTPATH environment
variable

(3) on MacOSX, update application to ensure DCMDICTPATH is set when
Slicer is started using Slicer.app bundle (in that case no launcher
is used)

This will allow for more robust interpretation of DICOM private tags
based on the information contained in the DCMTK private dictionary.

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24473 3bd1e089-480b-0410-8dfb-8563597acbee