Still a lot more to do but pushing WIP to not have it wasted if more
commits come in from others.
Also got rid of some global variables.
(cherry picked from commit ff2d22d915)
None of our load plug-ins have such a checkbox, so this is not
consistent. Moreover one you uncheck it, you just can't get back the
dialog until next GIMP restart. That's very bad usability.
(cherry picked from commit 948608e658)