Add new request::urgent signal

This fix two things:

(1) Clients asking to be urgent while focussed, this have been reported
a few time for urxvt and I usually link a patch that fix this. This may
not be considered a bug by some, but I think it is.

(2) Add the ability to stop noisy clients from setting the urgent state
themselves.
Add 'urgent' property to tags
Use tag urgent::property instead of looping every clients in awful.widgets.taglist