Item Detail panel handlers should be themeable

It's not possible to override the HTML output of the Item Detail Panel's individual items, because the handlers do not call theme functions.

Here are the current handlers:

amazon_store_participant_format
amazon_store_manufacturer_format (this one calls a theme function, 'amazon_store_search_results_manufacturer')
amazon_store_binding_format
amazon_store_feature_format
amazon_store_dimensions_format

This might be good for efficiency, but it makes it impossible to generate custom markup.