allow qt graphics toolkit to be used in --no-gui mode (bug #44116) * octave-gui.cc (octave_start_gui): Install Qt plot functions and register the qt plot toolkit here. * main-window.cc (main_window::construct): Not here.

%!test
%! warning off Octave:function-name-clash
%! f = testclass (1, 2);
%! assert (one (f), 1);
%! assert (two (f), 2);
%! rehash ();
%! assert (one (f), 1);
%! assert (two (f), 2);
%! assert (two (f), 2);