* libgimpwidgets/gimpzoommodel.[ch]: use private data instead of
  struct members. Added min-factor and max-factor properties so we can
  set the zoom range. These can be set by a new utility function:
  gimp_zoom_model_set_range.

* libgimpwidgets/gimpwidgets.def: added gimp_zoom_model_set_range.

* libgimp/gimpzoompreview.c: use gimp_zoom_model_set_range so we can
  only zoom in.