Instead of passing it around or fetching the singleton, keep a pointer
to the monitor manager that owns the output. This will eventually be
replaced with a per GPU/graphics card object.
https://bugzilla.gnome.org/show_bug.cgi?id=785381
Turn MetaOutput into a GObject and move it to a separate file. This
changes the storage format, resulting in changing the API for accessing
MetaOutputs from using an array, to using a GList.
https://bugzilla.gnome.org/show_bug.cgi?id=785381