- Move the property widget functions for GimpResource properties into a new
  libgimp/gimppropwidgets.[ch] file. This mirrors the files
  libgimpwidgets/gimppropwidgets.[ch] which are for more generic property types.
- Rename the functions gimp_prop_chooser_*_new() to gimp_prop_*_chooser_new().
- gimp_prop_chooser_factory() doesn't need to be public.
- Add a label to GimpResourceSelectButton, make so that the
  gimp_prop_chooser_*_new() functions set the property nick to this label and
  add this label to the size group in GimpProcedureDialog.