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.
Just like 5 years ago, the dependency mess caused by the giant
`awful.client`, `awful.tag` and `awful.placement` requires to
split the code into small files with less dependencies and include
those.
In this case, the goal is to use the `awful.mouse.client` functions
from `awful.client`.
Emmanuel Lepage Vallee