BUG: Fix/improve launcher path setup

Take advantage of recent changes in the CTK AppLauncher to use the list
form of setting up additional environment path lists. Particularly, we
now declare as path lists those environment variables that are (i.e.
QT_PLUGIN_PATH, PYTHONPATH) so that we no longer lose the user's paths
that have been set in these.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23036 3bd1e089-480b-0410-8dfb-8563597acbee