Support notify_notification_add_action ()

[arshubham]

It seems notify_notification_add_action () is not supported by this crate.
https://developer.gnome.org/libnotify/0.7/NotifyNotification.html#notify-notification-add-action

[hasufell]

It seems it was commented out https://github.com/hasufell/rust-libnotify/blob/master/libnotify-sys/src/lib.rs#L66 not sure why. Do you want to enable it and test?