Serialization of the current selection, when that selection crosses shadow roots

[keithamus]

WCCG had their spring F2F in which this was discussed. Present members of WCCG identified an action item to raise this issue. You can read the full notes of the discussion (WCCG had their spring F2F) in which this was discussed, heading entitled "Selection API". I won't be quoting the notes for brevity.

How do cross root trees that art part of the current selection get serialized?

Refs #161