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]
add "always" defeat_condition
rename remove_from_carryover_on_leaders_loss_

to remove_from_carryover_on_defeat_
Merge pull request #165 from gfgtdf/defeat_condition

 replace fight_on_without_leader with defeat_condition