Spawning Yoleck entities during gameplay

[idanarye]

In order to save entities (#23) they need to be Yoleck entities, which currently means they have to be created from the .yol file. But what if we want to save entities created at runtime? For example - in a strategy game the player will build structures, and we want to save these structures even though they are not in the .yol file.

To support this, we need to be able to generate Yoleck entities during gameplay.