power: add a method to retrieve the backlight output id

We're going to use this later.

https://bugzilla.gnome.org/show_bug.cgi?id=704086
power: return output id when changing screen brightness

When StepUp() or StepDown() are called to change the screen brightness,
also return the output id where the brightness transition happened (if
any).
This is useful so that the shell can show the OSD only on the relevant
monitor, which will be done in the next commit.

https://bugzilla.gnome.org/show_bug.cgi?id=704086
media-keys: pass output id to show_osd()

This is currently used by the brightness change OSD.

https://bugzilla.gnome.org/show_bug.cgi?id=704086