Some minor code style issues remained 
that I missed in 8adcc0cd.
After further reflection, I also converted
the code to be a private/internal function.
gimp_prop_check_button_new () should
cover the majority of GtkCheckButtons,
and the function is currently only used
to fix the size of those widgets. We could
revisit it as a public function in the
future if more use cases are found.