I got rid of it because Visualizers are not general purpose objects and their lifetime is completely managed by `DataSourceDisplay`.  I could have renamed `destroy` to something else, but I didn't see the point.  Adding `isDestroyed` here feels like consistency for the sake of consistency taken to far with no real benefit.

Now that's I've said my peace, if I didn't convince you I can put it back; but it just feels pointless.