Make sure that termonad can see GTK icons.

This uses wrapGAppsHook to be able to make it so that Termonad can see
the GTK icons it uses.

This is as described in NixOS/nixpkgs#43150.

It appears that hicolor-icon-theme doesn't actually contain any icons,
so instead I'm using adwaita-icon-theme.

.nix-helpers/termonad-with-packages.nix

@@ -105,17 +105,28 @@ in
 with (import ./nixpkgs.nix { inherit compiler nixpkgs; });
 
 let
-  ghcWithPackages = haskellPackages.ghcWithPackages;
-  env = ghcWithPackages (self: [ self.termonad ] ++ extraHaskellPackages self);
+  env = haskellPackages.ghcWithPackages (self: [
+    self.termonad
+  ] ++ extraHaskellPackages self);
 in
 
 stdenv.mkDerivation {
   name = "termonad-with-packages-${env.version}";
-  nativeBuildInputs = [ makeWrapper ];
+  buildInputs = [ gnome3.adwaita-icon-theme hicolor-icon-theme ];
+  nativeBuildInputs = [ wrapGAppsHook ];
+  preFixup = ''
+    gappsWrapperArgs+=(
+      # Thumbnailers (this probably isn't actually needed, but it is a good example of how to use --prefix)
+      --prefix XDG_DATA_DIRS : "${gdk_pixbuf}/share"
+      --prefix XDG_DATA_DIRS : "${librsvg}/share"
+      --prefix XDG_DATA_DIRS : "${shared-mime-info}/share"
+      --set NIX_GHC "${env}/bin/ghc"
+      )
+    '';
   buildCommand = ''
     mkdir -p $out/bin
-    makeWrapper ${env}/bin/termonad $out/bin/termonad \
-      --set NIX_GHC "${env}/bin/ghc"
+    ln -sf ${env}/bin/termonad $out/bin/termonad
+    wrapGAppsHook
   '';
   preferLocalBuild = true;
   allowSubstitutes = false;