[FEATURE] Refactor \Knp\DoctrineBehaviors\ORM\Tree\Tree::getRootNodes to support QueryBuilder customization

Split \Knp\DoctrineBehaviors\ORM\Tree\Tree::getRootNodes into \Knp\DoctrineBehaviors\ORM\Tree\Tree::getRootNodesQB and \Knp\DoctrineBehaviors\ORM\Tree\Tree::getRootNodes to support custom queries.