replace fight_on_without_leader with defeat_condition

[side] now has an attribute defeat_condition which can be "no_leader"(default), "no_units" or "never"
"no_leader" behaves liek it behaved before.
"no_units" behaves like fight_on_without_leader=yes behaved before
"never" is new and causes the side to never get defeated, this usually means the scenariodesigner has to end the scenario with [endlevel]

Conflicts:
	src/play_controller.cpp
	src/team.cpp
add "always" defeat_condition

Conflicts:
	src/play_controller.cpp
rename remove_from_carryover_on_leaders_loss_

to remove_from_carryover_on_defeat_
rename values of [side]defeat_condition=

most likely defeat_condition= will re renamed too defeat= too.
add "write_to" parameter to wml_message

"write_to" lets the wmldev specify to where the message should be
written. Defaults to the previous behaviour.

The intention is for example that one might want to give a summary  as a
chat message and a very detailed information in the stderr file.
replace write_to with to_chat in [wml_message]

as shadowm suggested, because we are never in the situation that we want
this information in the chat but not in the stderr.
#include <map.hpp> -> #include "map.hpp"
don't load null hotkeys

this prevents a possible segfault because otherwise add_hotkey calls hotkey::set_scope_active(new_scope) where new_scope is SCOPE_COUNT.