Review - Split the mission class

[XavierAntoviaque]

Copying from the email reviews to make it easier to follow up on code reviews. I'll create a few bugs on the same principle. This one is targeted for the next version.

the Missions class needs to be split between a Mission class
(that is subclassed by the actual missions classes, such as
FirstMission) and a MissionEngine class, that is responsible for
instantiating mission objects, saving/restoring them (methods such as
save_state()) or calling the right method corresponding to an action
(process_action_queue).

=> this will allow to only expose methods in Mission that can actually
be needed by someone scripting a mission, not the underlying magic. It
also gives more flexibility, allowing a player to run several missions
at the same time for example