Motivation for this change

QSynth 0.5.2 fails to compile for me now, presumably because of the incompatibility with fluidsynth >= 2.0.0.

The last version is actually 0.5.5 but I didn't manage to compile it because of this issue.

The previous issue with the desktop file was more "moved" than fixed ^^

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.