Handle albums with multiple CDs/disks correctly

[cpieloth]

• handle albums with multiple CDs/disks correctly
• an album with multiple CDs/disk can have
  • one album folder without sub-folders and disk number in ID3 tag
  • one album folder without sub-folders and disk number in file name
  • one album folder with a sub-folders per disk
• all files of one album with multiple disk must be concat in correct order, first CD1, track 1..., CD2, track 1... and so on