MRView ROI editor: fix edit button behaviour
MRView ROI editor: fix edit button behaviour

Fixes #301 - the edit button now unticks when creating or opening a new
RI.

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