In order to go back in monitor configurations, save them to a history.
The history is implemented as a max 3 element long queue, where newly
set configurations are pushed to the head, and old are popped from the
tail.

The difference between using a single previous config reference and a
queue is that we can now remember the configuration used prior to a
D-Bus triggered configuration when the user discarded the configuration.

This will later be used to restore a previous configuration when a
laptop lid is opened.

https://bugzilla.gnome.org/show_bug.cgi?id=777732