MonitorManager: add support for backlight

GnomeRR needs that too.
The backlight is exported as a normalized 0-100 value, or -1 if not
supported. Clamping to HW limits is handled by the backend.
Changing backlight uses a different method call, to avoid recomputing
the full display configuration every time the user presses the
backlight keys.

https://bugzilla.gnome.org/show_bug.cgi?id=705670