+ jACT-R Log view: allow columns to be shown/hidden using the view's
menu (visibility is stored in preferences not available on the
preference page)
+ jACT-R Log view: cells of the PRODUCTION stream only show the last
line of stream data (i.e. which production fired)
+ Buffer State view: expand/collapse state of tree nodes is retained
over multiple points in time selected in the jACT-R Log view
+ jACT-R Visicon: added a zoom slider