Bugfix: Ensure we never lose a view during a drop.
Bugfix: Ensure we never lose a view during a drop.

* Since we perform a move view as a remove + insert, we may lose the
  target container during the "remove" operation. The "insert" operation
  would then fail because the target container has been removed from the
  DOM tree. The fix is to pick a "backup" target to use during "insert"
  if we run into this special case.

umop/axiom_old_private,chromium/axiom,mcanthony/axiom,ussuri/axiom,rpaquay/axiom