BUG: Fix force build for Slicer

Replace the 'build depends on always run step' rule for Slicer (which
does not work with at least Ninja) with a rule to remove the stamp for
the build step. This more reliably ensures that the build step for
Slicer is always executed.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23141 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Fix volume rendering object factory on VTKv6

Issue #3514

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23142 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Fix VTK python library paths on Windows for VTKv6

Issue #3514

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23143 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: Remove error message when loading volume with VTK6

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23144 3bd1e089-480b-0410-8dfb-8563597acbee