BUG: Make sure the segment selector widget combobox is updated

The checkable combobox was not repainted unless it was the first segment whose visibility was changed, otherwise it was only updated on mouse hover. Now repaint is forced when selection is changed to make sure the changes are reflected in the checkable combobox.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25424 3bd1e089-480b-0410-8dfb-8563597acbee