Whilst trying to get multiple selections working in VMMakerTool I
realised that PluggableListMorphOfMany is not pluggable for the getting
and setting of the parallel list of booleans that define the selected
items. This means I can't make two or more lists be multiple selection
if they share the same model.

Unless somebody has a fix for this lying around I'll make a pass at it
later. I guess the simplest thing is to add a couple of selectors but I
have this urge to make something somehow neater. Not sure how right now....

tim