To facilitate listening for action on arbitrary file descriptors,
I've added a GNotifier class. It's quite simple but very useful:
GNotifier notifier(fd, GNotifier::Read);
notifier.on_ready_to_read = [this] (GNotifier& fd) {
// read from fd or whatever else you like :^)
};
The callback will get invoked by GEventLoop when select() says we
have something to read on the fd.
