It'd be nice to have a function to get all the items from a collection. Such function can be used for calculating the save size and for exporting items from a collection.

A tentative name could be 'photos_query_collection_items_job_run' which should get as parameter the name of the collection. The _finish function would then return the GList of PhotosBaseItem with the items in the collection.

The challenge of adding such a function is dealing with the use of the current state context in the QueryBuilder code. The easiest solution is to create a new SearchContext inside photos_query_collection_items_job. The "proper" solution is to take the dependency of search context out of the QueryBuilder.