Add an "expanded-changed" signal to GimpViewable, which should be
emitted by subclasses when the viewable's expanded state changes.
Emit this signal when the expanded state of group layers changes.
Respond to this signal in GimpContainerView, by calling a new
expand_item() virtual function.  Implement expand_item() in
GimpContainerTreeView, expanding or collapsing the item as
necessary.