+'ConvertContent': Called by AbstractContent::convert when a conversion to another
+content model is requested.
+$content: The Content object to be converted.
+$toModel: The ID of the content model to convert to.
+$lossy: boolean indicating whether lossy conversion is allowed.
+&$result: Output parameter, in case the handler function wants to provide a
+converted Content object. Note that $result->getContentModel() must return $toModel.
+Handler functions that modify $result should generally return false to further
+attempts at conversion.
+