Fix a bug caused by the recent SourceWidget changes.

 * Now that the SourceWidget has a built-in viewport origin setting,
   scrolling the view does not cause mouse-move events.  (Previously,
   scrolling moved the widget-relative position, so apparently Qt sent a
   move event.)  Without the move events, the selection and hover are not
   updated.  Add a call to updateSelectionAndHover to fix this.