The control center will call this method when the configuration dialog
for a display opens/closes, which will cause the same labels to quickly
fade out and in again, looking like it's flickering.
This commit fixes the issue by removing the tweens altogether.
https://bugzilla.gnome.org/show_bug.cgi?id=751599
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
