GPII-364: The contrast preview now belongs to contrastTheme.
And not to contrastEnabled, in order to retain semantic consistency with
enactor performing the previewing.
GPII-364: Added "Contrast options" heading.
Had to do that in a listener and not in the protoTree because protoTrees
don't merge and i was overriding "fluid.prefs.panel.contrast"'s
protoTree.
Corrected a preview container layout issue.