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.
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.
This is the code from the 'gimp-2.9' branch of the plug-in, integrated into GIMP's build system.
