DPMS is configured from a bit all over the place: via D-Bus, via X11 and
when reading the current KMS state. Each of these places did it slightly
differently, directly poking at the field in MetaMonitorManager.

To make things a bit more managable, move the field into a new
MetaMonitorManagerPrivate, and add helpers to get and set the current
value. Prior to this, there were for example situations where the DPMS
setting was changed, but without signal listeners being notified about
it.

!506