Complete 'parents' and 'getTreeByLevel' methods and test cases.
Refactor 'parents' and 'getTreeByLevel' methods.
Refactor 'addChild' method. Add 'addChildForcibly' method.
Update property declaration.
Refactor get menu by name.
Refactor 'getMenuByName' method.
Add 'delete' method and test case.
Add property '_parent' and refactor 'parent' method.
Complete 'delete' method with keep children.
Remove unused comment.
Add return value.
Complete 'deleteWithChildren' method and test cases.