attempt to properly split up the four available Sigil PWADs

Add support for the registered version of Sigil, which contains two additional PWADs containing the exclusive MP3 soundtrack by Buckethead, namely sigil_shreds.wad and sigil_shreds_compat.wad for use with sigil.wad and sigil_compat.wad, respectively.

Install each PWAD into its own package, so users are able to play Sigil either as Episode 5 in source ports that support this or as an Episode 3 replacement in every other source port - with or without the MP3 soundtrack - depending on what's available and on personal taste (the MIDI soundtrack included in the regular PWADs isn't bad, btw!).