@Override
	public void addBundleListener(@Nonnull BundleListener<I> listener) {
		listeners.add(listener);
	}
History contains a stack of prior states of items.
If an item has not been modified there is no entry in this map.

@return Map of historical states of items within this bundle.
recaf-core/src/main/java/software/coley/recaf/workspace/model/bundle/BasicBundle.java