Fixed two workbench resize issues

The symptom in both cases was that it became impossible to scroll all the way
to the very bottom of panel content that was in a scroll panel.

1. unnecessary explicit sizing of HeaderPanel's content area in WorkbenchLayoutImpl
   was making the content area 10px too tall (overlapping the footer or running past
   the bottom of the screen). Now allowing automatic sizing.
2. automatic sizing of the content area in ListBarWidget was making the content area
   40px too tall (the size of the list bar header). Now the content sizing is being done
   explicitly in this widget.