gs-application: Change when withdraw possibly obsolete notifications
Withdraw possibly obsolete notifications only after the app is registered on the D-Bus, otherwise a runtime warning is shown on the console:
GLib-GIO g_application_get_dbus_connection: assertion 'application->priv->is_registered' failed
This is a follow up fix for !1355 (merged)
I do not know how I could overlook such obvious thing... possibly wrong testing on my side, I'm sorry.