Improve scene format

[RiscadoA]

Problem

Our current scene format isn't well-fitted for an editor. The distinction between the scene/sub-scene hierarchy and the child of hierarchy is hard to understand and represent.

Solution

We should change our scene format to something more standard, which considers the ChildOf relations.

My and @jdbaracho's proposal is the following format:

{
  "components": {
    "Car": {
      "speed": 100
    }
  },
  "entities": {
    "left-spot-light": {
      "components": {
        "cubos::engine::SpotLight": {}
      }
    },
    "right-spot-light": {
      "components": {
        "cubos::engine::SpotLight": {}
      }
    }
  }
}

Each scene essentially describes an entity and it's children. One confusing weird part of this are relations. I think they could be placed in an extra field, alongside "components".

{
  "relations": {
    "foo@cubos::engine::SpringConstraint@bar": {
      "strength": 0.4
    }
  },
  "entities": {
    "foo": {}
    "bar": {}
  }
}

Imports could also be placed in an extra field, "imports".

{
  "imports": {
    "subscene": "/assets/subscene.cubos"
  }
}

Given that we're giving ChildOf so much importance, it might make sense to make ChildOf a core relation, instead of part of a plugin.