Adds the subobject part to the sort key.

Based on the patch https://gerrit.wikimedia.org/r/#/c/70047/4

This needs some test, but before that someone needs to explain what this achieves and why this is the desired way to do it. Because I have just c/p stuff from the original patch.
Looking again at this change I think this is wrong and needs to be marked invalid. values for property _SKEY should always be of type SMWDIBlob as per https://github.com/SemanticMediaWiki/SemanticMediaWiki/blob/master/includes/DataTypeRegistry.php#L386
                        '__key' => DataItem::TYPE_BLOB, // Sort key of a page

Instead the existing code looks wrong.  getPropertyValues() could (and in most cases would IMHO) return an empty array and we seem to be checking an instance of on end(array())?