[Request] Add zoom button to viewer for each item

[mradamcox]

Describe the feature or enhancement you're looking for
It would be good to be able to zoom to the extent of a given item's mosaic in the main viewer for its locale.

If this request is linked to a problem, please describe.
Currently, the "full extent" is calculated as an aggregation of all items, using the extent of their key map. This isn't especially helpful for multi-volume editions, where each volume covers a different part of a city.

Additional context
The extent should not be based on the key map, only on the main layers content for the item.