Fix `darkroom-tentative-mode'

* darkroom.el (darkroom--margin-factor): New variable.
(darkroom--set-margins): Consider `darkroom-margin-factor'.
(darkroom--reset-margins): Always reset margins to 0.
(darkroom-increase-margins): Implement with `darkroom--margin-factor'.
(darkroom--enter): Renamed from `darkroom--turn-on'. Don't afect hooks here.
(darkroom--leave): Renamed from `darkroom--turn-off'.
(darkroom--enter-or-leave): New function.
(darkroom-mode): Use `darkroom--enter' and `darkroom--leave'. Set
window change hook here.
(darkroom--maybe-enable): Removed.
(darkroom-tentative-mode): Redesign.

* darkroom.el (darkroom--margin-factor): New variable.
(darkroom--set-margins):