Specify (but do not implement) the context parameter to toDataElement()

This allows the converter to provide the node handlers with context
information, which hybrid nodes (such as alien and meta) need to decide
which shape to take.

Change-Id: I36860bee560a38ee39a149109be3706e39258edc
Hybrid-ify AlienBlock/AlienInline

Change-Id: Ieda8383c0c04df208f4c5f2aa7380427467049b7
Hybrid-ify MetaBlock/MetaInline

Change-Id: I7b7a2d50637e2009742b290cb1bd0f4fc0395934
Use AnnotationSet rather than array in getDataFromDom()

Change-Id: Ie5f1f94bd62739ccf74c027dd0ba418fe2d91fa6
Merge changes Ie5f1f94b,I7b7a2d50,Ieda8383c,I36860bee

* changes:
  Use AnnotationSet rather than array in getDataFromDom()
  Hybrid-ify MetaBlock/MetaInline
  Hybrid-ify AlienBlock/AlienInline
  Specify (but do not implement) the context parameter to toDataElement()