Add layout update hande information.

[drew7721]

Many blocks are added, moved and changed in multiple layout update handles.
It would be nice to know the one that actually rendered the template.

For instance, there are many catalog_product_view layout handles that get merged together to create the layout. I wonder if there is a way to know which one had the last word on the block. This might not be possible so it needs to be investigated.

It would, however, be nice to know at least that it was the catalog_product_veiw handle that generated the block. Especially if the block comes from a custom layout update handle.

[drew7721]

A block with the update handles is added at the bottom of the page.