WIP: Be more crispy-doom friendly

Hi,

I started this because I find frustrating that the .desktop files for some Doom related packages refers to 'boom' instead of 'doom', and that they doesn't appear in my list of games in gnome-shell if I only installed 'crispy-doom' and not prboom nor doomsday.

I digged a little bit more & fixed a thing or two in the "Doom II Masterlevels" launcher.

I know this is suboptimal, but "it works for me", so here is the results.