Add an 'As Animation' toggle to the GIF export options. If checked,
the GIF will be written as an animation. Only enable it on images with
more than on layer though. Add the new GIFSaveVals as_animation member
last so we don't break gimp_get_data() of old data. The fix moves the
GIF options fetching to *before* gimp_export_image(). If as-animation,
we simply pass GIMP_EXPORT_CAN_HANDLE_LAYERS to capabilities so the
layers are kept.