In GimpImage, update the image's bounding box in response to the
layer container's "add" and "remove" signals, instead of during
gimp_image_{add,remove}_layer(), so that the bounding box is
properly updated when moving an existing layer inside/outside of a
layer group, instead of only when adding/removing a new layer.

Even though moving a layer across group boundary doesn't change the
overall image bounding box, it does change the group's bounding
box, affecting the image bounding box.  It's therefore necessary to
update the image bounding box again when the layer is re-added to
the layer stack, so that the bounding box doesn't get stuck in an
intermediate state.