Entity prototypes for creating entities

I propose a new mechanism for creating entities by using "entity prototypes". A prototype is a "recipe" for an entity and optional child entities.
Currently some of the older entities (like "house3") contain logic in their creation script for creating child entities. This is not something which should be connected that closely to the entity, and should be decoupled.
Another example of where this would be useful is with spawn points, which spawns new entities. Currently we use a deep type tree, with classes like "settler", "merchant" etc. These types should be removed in favour of a single "human" type, and the different types should be created through prototypes.

A prototype should contain atlas data for populating new entities with both properties and sub entities.