BUG: handle missing parameter for propagationMode for legacy mrml files

Ron reported that the editor did not display correctly when
loading an old mrml file (the SPL brain atlas from the
DataStore).

The issue was that the editor parameter node in the file did not
contain a value for the propagationMode field and so the editor
interface could not be constructed.

This fix ensures that a valid propagation mode is selected in
this case.

TODO: it would be good to have a sanity check on the node to confirm
that all needed values exist and if not they should be set to
the application default.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24510 3bd1e089-480b-0410-8dfb-8563597acbee