Allow nodes to handle their own children

For data->DOM, this is easy: .toDataElements() can optionally return an
array instead of an object, and that will be treated as the data to
insert. If this happens, the converter won't descend. The node handler
can recursively invoke the converter if it needs to (although I suspect
the current implementation is broken when converting block content in an
inline context).

For DOM->data, this is a bit more complex. The node sets
.static.handlesOwnChildren = true; , which triggers the converter to
pass a data slice rather than a single data element, and not to
descend. The node handler can invoke the converter to recursively
convert DOM subtrees to data.

ve.dm.Converter (data->DOM):
* Renamed createDataElement() to createDataElements()
** .toDataElement() may return element or array, handle this
* Renamed childDataElement to childDataElements, is now an array
* Actually alienate if .toDataElement() returns null
** Shockingly, this claimed to be supported before but wasn't
* Rather than pushing to data, concat to it
** Add closing if needed
* Don't descend if .toDataElement() returned an array of length >1, or
  if the node has .handlesOwnChildren = true

ve.dm.Converter (DOM->data):
* Split getDomSubtreeFromData() and getDomFromData()
* When converting a node that handles its own children, pass in a data
  slice and skip over that data

Change-Id: I196cb4c0895cbf0b428a189adb61b56565573ab3