[gui] recompute project menu after loading projects

incidentally, this avoids having the phantom duplication of current project
when duplicating a non-current project.

src/plugins/gui/project_manager.ml

@@ -274,9 +274,12 @@ let () =
          let filter p' = not (Project.equal p p') in
          recompute ~filter window menu
        in
+       let hook () = recompute window menu in
        Project.register_create_hook callback_prj;
        Project.register_after_set_current_hook ~user_only:true callback_prj;
        Project.register_before_remove_hook callback_rm_prj;
+       Project.register_after_load_hook hook;
+       Project.register_after_global_load_hook hook;
        recompute window menu)
 
 (*