Reimplement tray icon support on top of NaXembed. NaTrayIcon now
becomes a subclass of this type, replacing GtkSocket. The
ShellTrayIcon wrapper now embeds directly a NaTrayIcon, the
ShellEmbeddedWindow GTK widget and ShellGtkEmbed ClutterClone
objects are no longer necessary to wrap a NaXembed socket window.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2590>
