Doom: more SIGIL

Before merging anything doom related, let's summon @fabian :-)

I would like to make a package doom-sigil-full for "SIGIL 1 with MP3 soundtrack", incorporating the free wad in it. The problem I've been trying to solve is that in the current state we don't get an icon for the desktop entry for doom-sigil-music (the python plugin tries to extract it from sigil_shreds.wad instead of sigil.wad). Also, I don't see the point of having 2 desktop entries, one for 'SIGIL free' and another for 'SIGIL MP3 soundtrack'. If someone bought the 'upgrade' I guess they'll want to use that.

In your opinion, is it OK to go that way ? or do you really care about the chain of expansion_for : doom-wad -> doom-sigil-wad -> doom-sigil-music and should I investigate to tries to make the plugin smarter and extract the icon from sigil.wad for the package doom-sigil-music

The second commit is less debatable I suppose, you have released a new crispy-doom version, let's make use of it ^^