BUG: 3416. Added generic mechanism for persisting node references in .mrml files. 
References stored in the form: references="role1:id1 id2 iid3;role2:id3;". Note that for backwards compatibility the  reference nodes such as displayable, transformable etc. still read/write their references the old way. Only custom references will use new syntax.


git-svn-id: http://svn.slicer.org/Slicer4/trunk@22610 3bd1e089-480b-0410-8dfb-8563597acbee