Add Nasal callback to build menubar

src/GUI/FGNasalMenuBar.cxx

@@ -18,6 +18,7 @@
 
 #include <Main/fg_props.hxx>
 #include <Main/globals.hxx>
+#include <Scripting/NasalSys.hxx>
 
 static bool nameIsSeperator(const std::string& n)
 {
@@ -275,6 +276,23 @@ void FGNasalMenuBar::init()
     configure(props);
 }
 
+void FGNasalMenuBar::postinit()
+{
+    auto nas = globals->get_subsystem<FGNasalSys>();
+    nasal::Context ctx;
+    nasal::Hash guiModule{nas->getModule("gui"), ctx};
+
+    using MenuBarRef = std::shared_ptr<NasalMenuBarPrivate>;
+    auto f = guiModule.get<std::function<void(MenuBarRef)>>("_createMenuBar");
+    if (!f) {
+        SG_LOG(SG_GUI, SG_DEV_ALERT, "GUI: _createMenuBar implementation not found");
+        return;
+    }
+
+    // invoke nasal callback to build up the menubar
+    f(_d);
+}
+
 void FGNasalMenuBar::show()
 {
     _d->visibilityMode = VisibilityMode::Visible;

src/GUI/FGNasalMenuBar.hxx

@@ -42,6 +42,8 @@ public:
      */
     void init() override;
 
+    void postinit() override;
+
     /**
      * Make the menu bar visible.
      */

src/GUI/menubar.hxx

@@ -30,7 +30,11 @@ public:
      * Initialize the menu bar from $FG_ROOT/gui/menubar.xml
      */
     virtual void init () = 0;
-    
+
+    virtual void postinit()
+    {
+    }
+
     /**
      * Make the menu bar visible.
      */

src/GUI/new_gui.cxx

@@ -269,6 +269,8 @@ void NewGUI::postinit()
     FGPUICompatDialog::setupGhost(compatModule);
     PUICompatObject::setupGhost(compatModule);
     FGNasalMenuBar::setupGhosts(compatModule);
+
+    _menubar->postinit();
 }
 
 void