Renames 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
Change uses of entity "owner" to "parent"
Merge pull request #436 from aledsage/refactor/rename-owner-to-parent

rename entity "owner" to "parent"