Fix #858: Selection: Avoid range confusion involving orphaned nodes

If an update occurs on the row at which a selection range was started,
and the user later shift+selects, an infinite loop occurs because
Selection wrongly assumes that the element from the starting row is
still in the document, when it was actually replaced with a new one.
This works around that by detecting if the element is no longer in
the document, and grabbing the equivalent element that is in the document
instead.
CHANGES.md: Note Selection orphaned range start row fix