CleanupAddon contained code to automatically make a UI element
visible when one of its children became visible. This is
appropriate for part stacks and sash containers but not for
toolbars, so we use instanceof checks to ensure we're using
it when appropriate.

Change-Id: I821b96ddffea10ed10840ed64fed7cc05e488bae
Signed-off-by: Stefan Xenos <sxenos@google.com>