When a layer is removed from a layer stack, and its backdrop needs
invalidating, emit the UPDATE signals for the backdrop only after
the layer has been removed from the container.
A subclass of GimpDrawableStack, for layer stacks. Invalidates the
layers' backdrop as/when necessary, according to the value of their
excludes_backdrop property.
Make gimp_drawable_stack_update() protected, instead of private, so
that we can use it in GimpLayerStack.
