- remove gimp_widget_flush_expose()
- remove the "now" argument to gimp_display_shell_flush() and make it
  only update widget states
- rename gimp_display_flush_whenever() to gimp_display_flush_update_region()
  and call gimp_display_shell_flush() separately in the only case we
  passed FALSE to flush_whenever()
- remove th flush_now interval logic from GimpDisplay, as soon as we
  have exposed the canvas, we are in the loop for the next frame clock
  tick anyway, so delaying a useless and removed process_updates serves
  no purpose
- in gimptool-progress.c, create the invisible grab widget also for
  non-cencelable cases, so we can always safely run the main loop
  manually to make the progress updates visible
- in gimp-gegl-apply-operation.c, always run the main loop manually
  to make the progress updates visible
- in gimpstatusbar.c, leave some FIXME comments as reminder that
  we might need the same logic as in gimptool-progress.c