XTREE enhancement: new PopupMenu commands:

$CBSETX: same as $CBSET but sets xtr'xvalidate bit &h08

$CBRSTX: same as $CBRST but sets xtr'xvalidate bit &h08

$CBUNDOX: same as $CBUNDO but sets xtr'xvalidate bit &h10

These new variations of the checkbox set, reset, and undo commands resolve a problem with the standard ones involving ambiguity over the possible interpretations of the xtr'validate = 3 that they return.