Premium-Midis become Pro-Midifiles

As we don't have any Midifiles left in the catalog, which are present only in the GM format, we only differ even if there are extensions for Korg and Yamaha (Pro), or not.

The article group called "Premium-Midi" is omitted and is now called Pro-Midifile.

The former Pro-Midifiles are now referred to only as Midifiles.