Make mapper node-independent

Ideally we could use FEMP for mapping not only to node-based feed items, but any feed items.

Once #397682: Let mappers declare mapping targets lands, we are a lot closer to independency from nodes, because detection of mapping targets will not be the responsibility of feedapi_mapper.module anymore.

What's missing then is wrapping the node object in a mapping target object:

// Simple mapper target object wrapper.
class feedapiMapperTarget {
  public $type; // e. g. 'node'
  public $object; // e. g. $node object
}

and modifying the mapping code accordingly. For instance, node mappers would not react to mapping objects that aren't of type node, RDF mappers would not react to mapping targets of type RDF.