World::debugDrawTarget property insteed of DebugDraw object

it's just a proposal to usage improvement. Since DebugDraw object is not configurable I suggest to turn it to the suitable property. Like that:
```javascript
World {
    id: physicsWorld
    debugDrawTarget: root
}
```
As for me I would prefer boolean `enableDebugDraw` but it's visual item and must have parent item in `QML` tree.