The monitor manager instance was created and setup in one step; at
construction. This is problematic if, in the future, the monitor manager
creation can fail, as the monitor manager is created quite late.
To make it possible to in the future fail creating a monitor manager,
create the instance very early when initiating the backend, then on
post init backend setup, "setup" the monitor manager state, i.e. read
the current state and setup the stage.
https://bugzilla.gnome.org/show_bug.cgi?id=785381
518fb9fb5e