mrview: changed tool order so that ROI layer is on top of overlay layer as discussed in #457
mrview: changed tool order so that ROI layer is on top of overlay layer as discussed in #457

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3