Implement SurfaceFragment.unwrapAllNodes and fix wrapAllNodes.

wrapAllNodes was calulating the new selection incorrectly. This has
been fixed and a test added.

unwrapAllNodes takes a depth as its argument and unwraps that many
elements from inside the selection.

Tests for wrap/unwrap apply also now check that applying a wrap
and then its inverse as an unwrap result in the document reverting
to its original state.

Change-Id: I7dcacdfb5894be59ffad69b369d7b32933a25b61
Merge "Implement SurfaceFragment.unwrapAllNodes and fix wrapAllNodes."