When a client is not visible, this would adjust its stacking order
still.
This also addresses https://github.com/awesomeWM/awesome/issues/472,
because it raises unminimized clients after they got focused.
This still does `client.focus = c` by default, but allows to customize
it.
This was initially suggested in #194, but by using `request::activate`
instead, which would not be the same. Therefore a new signal is being
used instead.
Helped-by: Samir Benmendil <samir.benmendil@gmail.com>