xfce: unbreak gtk3 theming for xfce desktop users

This is a partial revert of f0da9ff4.

The half-usable menus in gtk3 apps have been aggravating me for months.

pkgs/desktops/xfce/default.nix

···
 
   exo             = callPackage ./core/exo.nix { };
   garcon          = callPackage ./core/garcon.nix { };
-  gtk_xfce_engine = callPackage ./core/gtk-xfce-engine.nix { withGtk3 = true; }; # ToDo: when should be used?
+  gtk_xfce_engine = callPackage ./core/gtk-xfce-engine.nix
+    { withGtk3 = false; }; # = true; was completely breaking GTK3 app layout
   libxfce4ui      = callPackage ./core/libxfce4ui.nix { };
   libxfce4ui_gtk3 = libxfce4ui.override { withGtk3 = true; };
   libxfce4util    = callPackage ./core/libxfce4util.nix { };