I was directed by Massimo to some bug which was repeatedly generating
dozens of thousands of GEGL WARNINGs and that was completely taking over
the GUI if redirected to GimpErrorDialog. Currently GEGL warnings are
not redirected there, but the problem is still there, and we don't want
GIMP warnings to freeze the whole GUI either.
So only increment the repeat variable upon gimp_message_box_repeat() and
delay actual GUI update to a later low-priority idle function.