Design and implement event system

[Witek902]

Goal features:

• Easy API for creating new event types.
• Custom, game-defined event types.
• Consider that a user may do anything in a event callback (for example destroy an entity - event listener, which will be referenced by a next event in the queue).
• The events API should be flexible. That means, that adding a new event type won't require modification in multiple files, but defining a new class or something.

See: http://bitsquid.blogspot.com/2009/12/events.html

Engine's built-in events:

• [ ] entity creation / deletion
• [ ] resource load / unload
• [ ] bodies collision
• [ ] scene segment switch
• [ ] triggers (entity enters or leaves a trigger area)