[Gtk] Menu: place it before toolbar

[lytico]

issue

if a Menu is activated after the Toolbar,
it appears under the Toolbar

hints

adding the MenuBar is done in

https://github.com/lytico/maui/blob/2dd6c244e49962085dbfe1274f0b54cb7c6c6ea3/src/Core/src/Handlers/Window/WindowHandler.Gtk.cs

https://github.com/lytico/maui/blob/f3115025f20296b558b831f856b105b0ac8fc7a7/src/Core/src/Platform/Gtk/WindowExtensions.cs

and at the end its added to

https://github.com/lytico/maui/blob/c46a28c62801519d44ac40df4fe14f8e3e00479a/src/Core/src/Platform/Gtk/NavigationView.cs

so implement something like IToolbarContainer: IMenuContainer