Windows: replace "Arial" by "Microsoft Sans Serif" for the FL_HELVETICA font family

Users who need the old behavior for strict backwards compatibility under Windows can load the previous (Arial) font at program startup with only a few lines of code:

#ifdef _WIN32
  // reset Windows fonts to pre-1.4.2 state
  Fl::set_font(FL_HELVETICA,     " Arial");
  Fl::set_font(FL_HELVETICA + 1, "BArial");
  Fl::set_font(FL_HELVETICA + 2, "IArial");
  Fl::set_font(FL_HELVETICA + 3, "PArial");
#endif