When looking up variable's units attribute string, check
for "unit" (case insensitive) attribute if "units" is not present.
When looking up variable's long name attribute string, check
for "title" (case insensitive) attribute if "long_name" and "standard_name"
are not present.
Bugfix: Selection status of plot window items in the Window menu not
updated correctly when user selected front window.
Bugfix: Layout of credits help window not right when window reduced
to half or less of original size.
Bugfix: Status bar at window bottom not painted (i.e., left transparent)
on non-Mac OSes.