Process annotations bottom-up rather than top-down in data->DOM

This means that instead of creating a DOM element for an annotation,
then appending stuff to that DOM element, we queue up the things to
append and only create the DOM element after we've built everything
that's going to be in it.

Most significantly, this moves the toDomElements() call to the close
function, which paves the way for passing in the annotation's contents.

Change-Id: I98a7d3ebb0f3eb8627c22348b48596906db2646e
Pass child DOM elements to annotations' toDomElements()

This is special-cased for annotations but should be supported for
all model types once the converter is rewritten to be bottom-up
entirely.

Add a toDomElements() stub to ve.dm.Annotation overriding the stub
in ve.dm.Model so there's a place for the new parameter to be documented.

Change-Id: Id81da87b8b83d556a3618cc6187b22443a1e37e6
Allow annotations to render nothing

This effectively unwraps the annotation. Annotations can do this by
returning an empty array from their toDomElements() function.

Right now this is only supported for annotations, but once the converter
is rewritten to be entirely bottom-up, this is trivial to support for
other model types, and could even be used to implement unwrapping of
wrapper paragraph.

Change-Id: Ia572fd0610afccccfe795c257c0de9d003330f13
Move getHashObject() from dm.Node up into dm.Model

This way annotations inherit it too.

Also add htmlAttributes to the hash, and implement getComparableObject()
in terms of getHashObject().

Change-Id: Iea905f2b430f8e51c6026065be17b89b9a03cfab
Add a node class for mw:Nowiki

These represent <nowiki> tags. If the user doesn't edit the text inside
the nowiki, we round-trip the <span typeof="mw:Nowiki"> wrapper cleanly,
but if they do edit it, we unwrap it. This then triggers re-escaping
in Parsoid, and prevents cases where the user edits the text to no
longer need escaping but Parsoid still wraps it in <nowiki> because
of the <span typeof="mw:Nowiki"> wrapper.

In order to detect whether the contents have changed, the nowiki
annotation stores a copy of its contents. To avoid infinite recursion,
we have to exclude this attribute for hash generation.

Bug: 47678
Change-Id: I2edc46b6d87d2f91e952efcb09c0edae5166958f
Merge changes I2edc46b6,Iea905f2b,Ia572fd06,Id81da87b,I98a7d3eb

* changes:
  Add a node class for mw:Nowiki
  Move getHashObject() from dm.Node up into dm.Model
  Allow annotations to render nothing
  Pass child DOM elements to annotations' toDomElements()
  Process annotations bottom-up rather than top-down in data->DOM