Fix the annoying Mac behavior that Ctrl+W (Cmd+W on Mac) in the diff window closes the current vistrail.  Instead, close the diff window and the parameter inspector and legend subwindows when Ctrl+W is hit.
Fix the annoying Mac behavior that Ctrl+W (Cmd+W on Mac) in the diff
window closes the current vistrail.  Instead, close the diff window
and the parameter inspector and legend subwindows when Ctrl+W is hit.

gui/vis_diff.py:
  - QVisualDiff.__init__: install event filter
  - QVisualDiff.eventFilter: new method, filter Ctrl+W shortcut
    override and close window
  - QVisualDiff.closeEvent: new method, on close, close the tool
    windows



celiafish/VisTrails,minesense/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,hjanime/VisTrails,hjanime/VisTrails,minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails,hjanime/VisTrails,celiafish/VisTrails,Nikea/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,minesense/VisTrails,VisTrails/VisTrails,minesense/VisTrails,hjanime/VisTrails,Nikea/VisTrails,Nikea/VisTrails,VisTrails/VisTrails