Add support for XDG_ACTIVATION_TOKEN/XDG Activation protocol in GAppLaunchContext
Reading https://gitlab.gnome.org/GNOME/gtk/-/blob/main/gdk/wayland/gdkapplaunchcontext-wayland.c, it feels like get_startup_notify_id
vfunc is supposed to set XDG_ACTIVATION_TOKEN on Wayland. In https://gitlab.gnome.org/GNOME/glib/-/blob/main/gio/gdesktopappinfo.c, it seems glib always tries to use the X11-specific DESKTOP_STARTUP_ID/desktop-startup-id environment variable/platform data what fails on applications following what desktop-entry-spec describes.