Levels as entities

[idanarye]

Currently Yoleck levels are loaded with a resource - YoleckLoadingCommand. It would be better if instead a level would be entities, and we could load/unload the level by placing components on them (or maybe just sending events?). The benefits would be:

1. Being able to load multiple levels at the same time. This is good for:
   • Level transition.
   • Having long lived entities (like the player?) in their own level.
   • Level layers.
2. We can place other components on the level entities as a solution for Editable level data #2.
3. If we have a meta-level containing the levels as Yoleck entities, we could use that meta-level as a solution for Support non-linear level structure #4.