The goal is to catch cases where the return value exists, but is
forgotten. There was a large enough number of them to turn this
into a real check. Initially, I just wanted to implement it to fix
the problems, then delete the code. But since this is so common, I
think it is worth the annoyance.
From now on, all core object will have their own rules. `awful.rules`
hardcodes some client specific code. All `rules` module have some form
of class specific code. This code will now be part of a new module
called `ruled`. Since a year or so, a lot of work has been done to
refactor the rules on top of the shared `gears.matcher` class. This way
there wont be as much duplication.