(bs--window-config-coming-from): Make frame-local.
(bs--restore-window-config): New function.
(bs-kill, bs-select, bs-select-other-window, bs-select-other-frame): Use it.
(bs--show-with-configuration): Save the window configuration as a frame local
var, and only if *buffer-selection* is not already visible on this frame.
(bs--window-for-buffer): Return as soon as a matching buffer is found.