Merge branch 'docking-stations' - bgo#663126

This lets docking stations work better in the light of plugging external monitors.

It used to be that a docked laptop, with a closed lid, would still keep the
LCD active, thus making it pretty useless with an external monitor.

Now, we disable the LCD if the laptop's lid is closed, as expected.
Additionally, to decide when to suspend the laptop, we do so if there
are no plugged or usable RANDR outputs.  For example, a docked laptop
with a closed lid and *no* external monitor has no usable outputs, so
it will get suspended.

This requires gnome-desktop 3.3.4 or later, as the semantics of
gnome_rr_screen_new() are slightly different there - now it returns a
singleton instead of a new object every time.