rename entity "owner" to "parent"

- Renames entity.getOwner to entity.getParent (and setter + clearOwner)
- Renames entity.getOwnedChildren to entity.getChildren
- Renames entity.addOwnedChild to entity.addChild (and removeChild)
- Deprecates the old methods
- Changes all usages of the deprecated methods (and constructor flag "owner") to the new "parent" version