MetaMonitorConfig: don't keep a previous config with the wrong outputs

We can only apply a configuration if its outputs match the connected
ones, so discard the current configuration if the set of output changes
(for example for hotplug), otherwise we will crash trying to apply
the bogus previous configuration.

https://bugzilla.gnome.org/show_bug.cgi?id=725637
MetaMonitorConfig: don't always restore the previous config when opening the laptop lid

Only do it if the current configuration was actually created
as the result of closing the laptop lid.

https://bugzilla.gnome.org/show_bug.cgi?id=725637