Change MetaNodes to MetaItems

Rather than meta-things being special kinds of nodes, they are now a
separate class of things (MetaItems) along with Nodes and Annotations.

* Created a generic ve.dm.MetaItem that meta items inherit from.
  There will be actual instances of this class as well in the upcoming
  meta group code.
* Renamed MetaNode to AlienMetaItem, MWMetaNode to MWMetaItem,
  'metaBlock'/'metaInline' to 'alienMeta'
* Created a MetaItemFactory, handle meta items in the ModelRegistry
* Kill ve.dm.Node.static.isMeta, now obsolete

ve.dm.Converter:
* Pass in the MetaItemFactory
* Look up data element types in the ModelRegistry rather than the
  NodeFactory, because they can be either nodes or meta items
* Document createDataElement() and make explicit that modelClass can be
  either a node or a meta item
* Handle meta items in getDataFromDom()
* In getDomFromData(), check the MetaItemFactory as well as the NodeFactory

Change-Id: I893709c6f3aa00f85c1b905b70f9f4e597bdeada
Merge "Change MetaNodes to MetaItems"