application: Use the new API to get the startup notification ID

application: Use the new API to get the startup notification ID

The DESKTOP_STARTUP_ID gets cleared early after the process is spawned,
meaning that it's too late at add_platform_data() to pick it up and send
it over to the primary instance, as it will be always unset at that point.

To solve this, we use the new gdk_display_get_startup_notification_id()
method to pull the startup notification ID for the application, if present,
out of the display and pass it over to that primary instance.

https://gitlab.gnome.org/GNOME/gtk/issues/1084