make direction_from_item() return the normalized item
avoid duplicate call to get_orderby_key()
introduce get_adjacent_items()
link to wiki page describing all the parameters that p2p_register_connection_type() accepts
introduce P2P_Direction_Strategy interface

The idea is that it's better to have classes that implement a
small interface, instead of classes that overwrite seemingly random
methods of a big parent class.
make get_prev() and get_next() return raw objects
use scb_list_group_by()
introduce P2P_List_Renderer class

It keeps all the code related to rendering lists of items in one place.
trigger warning if first post type not found

see #349
extract method: expand_shortcuts()
extract method: expand_ctypes()
extract method: finalize_query_vars()
make create_from_qv() return the altered query variables
fix core strict standards errors