app: respond to viewable expanded state changes in container views
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.
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