Make sure all IDL members have "dfn" in prose #397

ReSpec shipped a new feature yesterday that warns about missing definitions in prose for IDL members.

ReSpec now reports 25 warnings on the Presentation API, some of them we should fix (e.g. binaryType, connection properties). Now, ReSpec also complains about the interface names themselves, where it makes sense to me to have the IDL be the definition, as done in the Presentation API. I filed an issue against ReSpec: https://github.com/w3c/respec/issues/997