Various IDE improvements

* The shared engine code from all 3 formats has been put in a single
file, 'engine.js', in the targets folder. This is transcluded into each
template at build time.
* All formats: <<elseif>> is recognised as an alternative to <<else if>>
* Added 'Twine on GitHub' Help menu item.
* Added 'Import From Compiled HTML' option, letting the author recover
passages from built games.
* Deleted TW and TW2 and all references to them.
* The delete dialog now only asks once for deleting multiple passages,
and notifies how many passage have links to the deleted passages.
* The Story Statistics dialog no longer records stats from passages that
aren't part of the story (scripts, etc).
* Removed the unused toRss() function from tiddlywiki.py.