Docs: State Qt5 as GUI requirement
Docs: State Qt5 as GUI requirement

The code will still compile with Qt4; this is only a change to the documentation to more strongly dissuade Qt4 usage, with the intention of removing Qt4 support in a future micro update.

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