Fix: The overflow calculation doesn't take into account the scrollbars

- Need to use clientWidth rather than offsetWidth. Apparently there is
  no jQuery equiv.
Dev fix: The previous commit which used the DataTables column-resize
event wasn't quite good enough. After that event has occured the scroll
draw in DataTables could still adjust the column sizes, which would
result in incorrect calculations here in FixedColumns. The fix is to
always calculate the column sizes on a redraw. That's fairly expensive
since this is DOM heavy, so I want to look at using css(width) which
should be faster, but we do need both inner and outerWidth