[ Enhancement ] Support global app menus on Wayland

[81reap]

I’d like Mango to support a global menu/app menu workflow similar to macOS or KDE Plasma’s Global Menu. I understand Mango itself doesn't have a menubar, but, if I understand correctly, this metadata would have to come from the window manager/compositor for any menubar to be able to read it.

I think for maximum compatibility Mango would need to support both of these protocols. That way KDE/Qt-style clients may use the KDE appmenu protocol and GTK apps may use gtk_shell1.

1. org_kde_kwin_appmenu_manager :: This lets Wayland clients associate a wl_surface with a DBusMenu service name + object path.

2. gtk_shell1 :: GTK clients can expose menu metadata through set_dbus_properties, including DBus menu paths.

Within Scope (estimate):

• Mango tracks per-surface appmenu metadata
• Mango updates that metadata when the focused toplevel changes
• Mango exposes the focused window’s menu endpoint to external consumers, either through a Wayland protocol path, IPC, or both
• an external panel can then render the DBusMenu for the focused app

Out of Scope:

• perfect compatibility with every GTK/Qt/Electron/XWayland app
• shipping a full menubar UI inside Mango itself

[81reap]

I raised a PR to add protocols for appmenu to work. This is my first time contributing to the repo, so if maintainers have any advice or feedback I would be all ears.

#833

thank you for your time