Move .commit()/.rollback() from TransactionProcessor to Document

Previously these were static functions in TransactionProcessor
which instantiated a TP called .process() on it. These are now
methods of ve.dm.Document.

Also moved the emission of the 'transact' event on the document from
TransactionProcessor to Document itself, and moved the tests asserting
double application is protected against from TP to Document (because
the corresponding code moved as well).

Change-Id: I7c9f22a14accaf0ba1f70d5aa4f0573bb7e677d0
Add mutators in MetaList

Add .insertMeta() and .removeMeta() methods to insert and remove
metadata through the meta list. For convenience, there is also a
.remove() method in MetaItem that wraps around removeMeta().

Also rename insertItem() and removeItem() to addInsertedItem() and
deleteRemovedItem() to avoid confusion, and make the MetaList
constructor take a dm.Surface rather than a document so we can call
change().

Change-Id: I4b62a3109404cfd56f5de68938e1db908b03e678
Merge changes I4b62a310,I7c9f22a1

* changes:
  Add mutators in MetaList
  Move .commit()/.rollback() from TransactionProcessor to Document