-No prefs so prefs calback not req'd;
-Remove (seemingly) not needed dissector registration on UDP port 0;
-Rework dissect_applemidi_heur() and related so that once dissect_applemidi_heur()
determines that a pair of UDP addr/ports are being used for the AppleMIDI protocol,
dissect_apple_midi() will always be called directly for UDP packets with that pair
(rather than dissect_applemidi_heur() always be called for every packet).
svn path=/trunk/; revision=31824