Ewmh urgent v1

Add better support for the "urgent" property for tags. It currently require too much code for every taglist implementations. This pull add the necessary code to better track urgent tags. It also add some code to prevent noisy clients from setting themselves as urgent when they have the focus. While there may be some good use case, I think it is usually unwanted.

Finally, I also add awful.tag.swap, umm, because I can!
It looks to me like emitting the signal could/should just get added to `client_set_urgent` for the `true` case?