Enable metadata for collection items
Key-based collections
Collection: Add metadata when adding item
Collection Controller: Collection items are unique
Content Handlers: Set priority when adding handler
Retrieve Content Handlers sorted by priority
Collection: Enable item querying (sorting)
Cache priority-sorted Content Handlers
Merge branch 'content_handler_prioritized' into develop
Prepare handler client output for attribute loading via AJAX
Load content handler attributes via AJAX
Merge branch 'content_handler_client_loading' into develop