Added scrollbar and min-width to workbench panels

 - When the screen resolution is low, workbench tools often go off the
   screen and there was no scrollbar to get at them.
 - When user resizes a workbench panel too small, contents of workbench
   tools get rearranged and look terrible, hence min-width.
Merge pull request #363 from jeremyherr/workbench-panel-css-fix

Added scrollbar and min-width to workbench panels