BUG: Ensure projections do not appear as selectable

Projections are not selectable and shouldn't be, however they
were displayed as if they were selectable, so it could be confusing
for the user. This fix ensure projections are properly displayed to
not look like selectable.

Fixes #3222

From: Laurent Chauvin <lchauvin@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22177 3bd1e089-480b-0410-8dfb-8563597acbee