remote-viewer: Add missing chain-up to parent's window_added() method
    
    It hasn't bitten us (so hard) so far but would from the moment we add
    support to app-menu/headerbar.
    
    By not chaining-up to the parent's method, gtk_window_set_application()
    is never called. This causes GApplication object not being able to load
    the app-menu, but using the "fallback" which only contains the "Quit"
    item.
    
    By chaining-up to the parent's method, g_application_hold() call on
    virt-viewer-app.c can be removed, as it is already called by the
    parent's window_added() method.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Acked-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>
    Acked-by: Jonathon Jongsma <jjongsma@redhat.com>