signal-desktop: Fix a possible crash when saving an attachment

If executed in a pure environment (nix-shell --pure) or depending on the
`gtk3` version of the system Signal-Desktop was e.g. crashing when
clicking on a PDF attachment (instead of showing the dialog to save a
file).
Using wrapGAppsHook and setting XDG_DATA_DIRS to the correct version
fixes this bug.

The error message was the following:
```
(signal-desktop:30756): Gtk-WARNING **: 14:04:49.073: Could not find the icon 'user-home-symbolic-ltr'. The 'hicolor' theme
was not found either, perhaps you need to install it.
You can get a copy from:
        http://icon-theme.freedesktop.org/releases

(signal-desktop:30756): GLib-GIO-ERROR **: 14:04:49.134: No GSettings schemas are installed on the system
Trace/breakpoint trap
```

pkgs/applications/networking/instant-messengers/signal-desktop/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, lib, fetchurl, dpkg, gnome3, gtk3, atk, cairo, pango, gdk_pixbuf, glib, freetype,
+{ stdenv, lib, fetchurl, dpkg, wrapGAppsHook, gnome3, gtk3, atk, cairo, pango, gdk_pixbuf, glib, freetype,
 fontconfig, dbus, libX11, xorg, libXi, libXcursor, libXdamage, libXrandr,
 libXcomposite, libXext, libXfixes, libXrender, libXtst, libXScrnSaver, nss,
 nspr, alsaLib, cups, expat, udev
@@ -52,7 +52,7 @@ in
         throw "Signal for Desktop is not currently supported on ${stdenv.system}";
 
     phases = [ "unpackPhase" "installPhase" ];
-    nativeBuildInputs = [ dpkg ];
+    nativeBuildInputs = [ dpkg wrapGAppsHook ];
     unpackPhase = "dpkg-deb -x $src .";
     installPhase = ''
       mkdir -p $out
@@ -67,6 +67,9 @@ in
       # Patch signal
       patchelf --set-interpreter "$(cat $NIX_CC/nix-support/dynamic-linker)" \
                --set-rpath ${rpath}:$out/libexec $out/libexec/signal-desktop
+      wrapProgram $out/libexec/signal-desktop \
+        --prefix XDG_DATA_DIRS : "${gtk3}/share/gsettings-schemas/${gtk3.name}/" \
+        "''${gappsWrapperArgs[@]}"
 
       # Symlink to bin
       mkdir -p $out/bin