Mission fix

[mrzzzrm]

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

[mrzzzrm]

(Please see this as an arch-proposal, nothing final. I'm fine with reworking it, completely differently, if necessary.)

[mrzzzrm]

The arch introduced here is obsolete by now, fix will follow in another PR