Add getDocumentSlice

A document slice is a document built from a data slice of an existing
document. It's completely separate from the original document and has
its own store and internalList. The new document's data also contains
the entirety of the original document's internal list. It's possible
to create a document slice of data located inside the internal list,
in which case the resulting document will contain that data twice (one
mutable copy at the top level, and one immutable copy in the internal
list).

ve.dm.Document.js:
* Optionally take an internalList in the constructor. This allows us to
  create a document with a clone of an existing internalList rather than
  an empty one.
* Add edgeMetadata flag to getFullData()

ve.dm.IndexValueStore.js, ve.dm.InternalList.js:
* Make these classes cloneable

Change-Id: I93e06f764ace16aee9df941b07f8c2bff1a28e2b
Merge "Add getDocumentSlice"