Removing the array part in field_mappings

Changing the storage format for the field_mappings in #__content_types to object->object instead of object->array[0]->object.
 The array part is not needed at all.

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30607

Same as https://github.com/joomla/joomla-cms/pull/1068, but with updated branch and able to merge.