Add wxTaskBarIcon::GetPopupMenu()

This is similar to CreatePopupMenu(), but the menu pointer returned by
the new function won't be deleted by wxWidgets, allowing it to return
the same pointer every time it is called.

Closes #18886.

src/common/taskbarcmn.cpp

@@ -53,6 +53,13 @@ void wxTaskBarIconBase::OnRightButtonDown(wxTaskBarIconEvent& WXUNUSED(event))
         PopupMenu(menu);
         delete menu;
     }
+    else
+    {
+        menu = GetPopupMenu();
+
+        if (menu)
+            PopupMenu(menu);
+    }
 }
 
 void wxTaskBarIconBase::Destroy()