Add an OSD monitor labeler exposed on DBus

Add an OSD monitor labeler exposed on DBus

This DBus API is intended to be used by gnome-control-center's
displays panel to show monitor labels.

Each output (i.e. hardware monitor) identified by its
org.gnome.Mutter.DisplayConfig API ID has at most one label. On
mirrored setups, all the labels for outputs corresponding to the same
logical monitor (i.e. showing the same contents in the same mode) are
shown together.

At most, only one DBus client at a time is allowed to show labels.

https://bugzilla.gnome.org/show_bug.cgi?id=743744