Multi-disc DiscSubtitle sorting

[simoncn]

It would not be necessary to use 1/3 to work out which DISCSUBTITLE the disc belongs to. All files in each disc are titled with DISCSUBTITLE, so this is sufficient.

The problem is knowing which order to display the discs. If disc 1 (or 1/3) shows up under more than one DISCSUBTITLE, MinimServer cannot know which of these DISCSUBTITLEs should display first. At present, the order of display is controlled by the DISCNUMBER tag.