Incremental enabling changes to allow searching of LiquidThreads by page and by thread:

Incremental enabling changes to allow searching of LiquidThreads by page and by thread:
* Add three new hooks, XmlDumpWriterOpenPage, ModifyExportQuery and XmlDumpWriterWriteRevision, to WikiExporter class.
* Hook two of these events to add a DiscussionThreading section to XML dumps, containing the parent, ancestor and discussion page to which a post belongs, if it is indeed a LiquidThreads post (as determined by joining on the thread table).
* Deprecate old calling style for Thread constructor, the $children parameter has been unused for yonks.