musescore: add darwin-specific package

This is basically a clone of the Homebrew cask version:
https://github.com/caskroom/homebrew-cask/blob/master/Casks/musescore.rb

pkgs/applications/audio/musescore/darwin.nix

@@ -0,0 +1,34 @@
+{ stdenv, fetchurl, undmg }:
+
+let
+  major = "2";
+  minor = "0.3";
+  patch = "1";
+  appName = "MuseScore ${major}";
+in
+
+stdenv.mkDerivation rec {
+  name = "musescore-darwin-${version}";
+  version = "${major}.${minor}.${patch}";
+
+  src = fetchurl {
+    url =  "ftp://ftp.osuosl.org/pub/musescore/releases/MuseScore-${major}.${minor}/MuseScore-${version}.dmg";
+    sha256 = "0a9v2nc7sx2az7xpd9i7b84m7xk9zcydfpis5fj334r5yqds4rm1";
+  };
+
+  buildInputs = [ undmg ];
+  installPhase = ''
+    mkdir -p "$out/Applications/${appName}.app"
+    cp -R . "$out/Applications/${appName}.app"
+    chmod a+x "$out/Applications/${appName}.app/Contents/MacOS/mscore"
+  '';
+
+  meta = with stdenv.lib; {
+    description = "Music notation and composition software";
+    homepage = https://musescore.org/;
+    license = licenses.gpl2;
+    platforms = platforms.darwin;
+    maintainers = with maintainers; [ yurrriq ];
+    repositories.git = https://github.com/musescore/MuseScore;
+  };
+}

pkgs/top-level/all-packages.nix

@@ -14542,7 +14542,11 @@ with pkgs;
       else null;
   };
 
-  musescore = libsForQt55.callPackage ../applications/audio/musescore { };
+  musescore =
+    if stdenv.isDarwin then
+      callPackage ../applications/audio/musescore/darwin.nix { }
+    else
+      libsForQt55.callPackage ../applications/audio/musescore { };
 
   mutt = callPackage ../applications/networking/mailreaders/mutt { };
   mutt-with-sidebar = callPackage ../applications/networking/mailreaders/mutt {