Also backtracked the removal of wesnoth.get_recall_units, which is now available as wesnoth.units.find_on_recall - it's just more convenient than passing x=recall to wesnoth.units.find
This removes the Lua deprecation_message function in favour of exposing the C++ variant to Lua instead.
It also moves all deprecation messages to a separate logdomain, making them easily enabled en masse.
(as proposed by @DeFender1031)
All existing deprecation messages in the Lua code have been changed to
use the new deprecation system.
The goal is to eventually use an equivalent system for ALL areas of the API.
