The ai is similar to the old c++ version. Only the target searching is
executed every 10 ticks only now to prevent performance issues with too
many lua calls.

Monsters can now either receive abilities at lifetime via scripts, or
via the <ability> node in the monsters node.

As a side effect i had to remove the monster attack AI for now. I will
readd this as next thing.

This allows to display hit messages in the client for abilities

Each ability can now define a cooldown that prevents the player from
using other abilities for a while. The time of this cooldown can be set
to any attribute. The modified value of the attribute is the value of
the cooldown in game ticks. The cooldown will be automatically started
if the ability has `autoconsume` set to true. Otherwise a script has to
call entity:cooldown_ability(ability).

You do not need to reset the mana in the scripts if you do not want to.
We will need this for attacks later since those do not nessecary have a
script.

Should allow easier modification without thinking of useful examples for
each one