ENH: adding SimpleFilters as internal modules via externalproject

Following the example of MultiVolumeImporter, a new external project
was added to download the SimpleFilters repository, the source
directory is propagated into the Slicer project and it is added as a
sub-directory to the build in scripted modules.

From: Bradley Lowekamp <blowekamp@mail.nih.gov>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22201 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: #2739: Add configuration of two slice viewers

Added layout with just two slice viewers arranged side by side

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22202 3bd1e089-480b-0410-8dfb-8563597acbee