Higher order relative LCRS support.

[softmarshmallow]

current relative LCRS retrieval only works for direct parent. if the text is in the box under the root box, which box is same size as text, and the box aligned lest to the root, text will return value of center, which we should provide a way to get relative LCRS to specific target (a.k.a root)

• root (root:box - left, root:text - left)
  • box (box:text - center)
    • text