Fix: if processor does not bear any settings, it may not be available from feedapi_get_settings(type, id). Make pulling of settings more robust.

#551444 - Dup $ sings to remove

#548852 by Alex Barth and Aron Novak - introduce feedapi_mapper_override() to handle operations on mappings correctly

Add feedapi_mapper_get_uniques() API function and expose _feedapi_mapper_get_feed_item_element() to third party usage as feedapi_mapper_get_element().

Simplify signature of feedapi_mapper_load_mapping().