Motivation for this change

My original motivation for this change was difficulty using dreamer/boxtron on NixOS as the fluid-soundfont was named FluidR3_GM2-2.sf2 instead of the more common FluidR3_GM.sf2.

What follows is an excerpt from Fedora's fluid-soundfont spec file that explains the situation pretty well:

# The original URL (http://www.powermage.com/fluid) seems dead. Therefore we point
# to the Hammersound archives:
URL:            http://www.hammersound.com/cgi-bin/soundlink.pl?action=view_category&category=Collections&ListStart=0&ListLength=20
# The Hammersound source gives us a soundfont in a linux-unfriendly .sfArk format. 
# In order to convert this to a linux-friendly .sf2 format one needs to use a 
# non-free utility sfarkxtc from 
#    http://www.melodymachine.com
# This page explains how this conversion is done:
#    http://vsr.informatik.tu-chemnitz.de/staff/jan/nted/doc/ch01s46.html
# Debian folks already did this and we will borrow their source tarball:
Source0:        http://ftp.de.debian.org/debian/pool/main/f/%{name}/%{name}_%{version}.orig.tar.gz 

So I've made a few changes:

• Update variables.
• Switch to Debian's mirror which includes both GM/GS soundfont versions.
• FluidR3_GM2-2.sf2 is now named to the more standard FluidR3_GM.sf2 and FluidR3_GS.sf2
• Update description, add longDescription.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @abbradar