Override which fields will be used for deduplication

Right now, it is hard coded in the processor which feed elements will be used for deduplication. E. g. FeedAPI Node processor, which produces nodes from feed items, assumes that if a new feed item's GUID or URL already exist, it is a duplicate.

This assumption is not always correct and sometimes very different fields should be used for determining what's a duplicate.

We could solve this problem by adding an option on the mapping form that allows to pick which fields should be used for deduplication. Processors like feedapi_node or others could then query feedapi_mapper to find out which fields should be used for deduplication.