- add gimp_image_get,get_xcf_compat_mode()
- add a compat toggle to GimpFileDialog which is shown and sensitive
  only for a save (not export), and if the image structure allows
  to save an old version at all. The button also has a tooltip
  which explains why it is sensitive and what it does
- add "gboolean xcf_compat" to file_save_dialog_save_image()
- in file_save_dialog_save_image(), call image_set_xcf_compat_mode(TRUE)
  only around the call to file_save() and set it to FALSE after saving
- in xcf_save_invoker(), honor the image's XCF compat flag and save an
  RLE-compressed XCF if possible

The above is very convoluted and doesn't pass the "xcf_compat" boolean
directly because we can't change the parameters of gimp-xcf-save, and
because the gimp-xcf-save might be called indirectly.