MRView: fix remaining deallocation issues
MRView: fix remaining deallocation issues

As discussed in #348. Once again, never work with raw pointers if you
can avoid it...

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