Get rid of 'reversed' flag on transactions

The way we implemented undoing transactions was horrible. We'd process
the original transaction, but with a reversed=true flag. That meant we
had to keep track of the 'reversed' flag everywhere, and use ternaries
like insert = reversed ? op.remove : op.insert; all over the place to
access transaction operations. Redo then worked by reapplying the
transaction. We would verify that this was OK by tracking whether the
transaction was in an applied state or an undone state.

This commit makes it so every transaction can only be applied once. To
undo, you obtain a mirror image of the transaction with tx.reverse(),
then apply that. To redo, you clone the original transaction with
tx.clone() and apply that. All the code that had to use ternaries to
check whether the transaction was being applied in reverse or not is
gone now, because you can only apply a given transaction forwards,
never in reverse.

Bonus:
* Make ve.dm.Document's .completeHistory a simple array of
  transactions, rather than transaction/boolean pairs
* In the protection of double application test, clone the example
  document properly; it modified ve.dm.example.data, which was "fine"
  because it ran .commit() and .rollback() the same number of times

Change-Id: I3050c5430be4a12510f22e20853560b92acebb67
Merge "Get rid of 'reversed' flag on transactions"