Mission fix

Fixes #564 

...by 
* Introducing WorldElement as a future base class of everything, for now only baseclass of Mission and Script
* Making WorldElement inherit from glow::Referenced and using glow::ref_ptr to keep Missions alive (as some kind of zombie, since they are not update()d anymore) in MissionStatePoll


This is a first step into the direction of what I imagine #542 to look like, but i'd rather deliver it in small bites than in one big PR. The following "issues" will for sure be removed:
* two update() loops in World::update()
* double constructors in Script