Background is monitoring the whole `org.gnome.desktop.background` gsettings keys
for changes connecting to the non-specialized 'changed' signal and re-emitting
this as-is.
This means that when the background is changed via control-center, we get
multiple 'changed' signal events from GSettings, and for each one of this we
recreate a Background and a BackgroundActor.
Avoid this by using an idle to delay the emission of the 'changed' signal
grouping the events.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/558
7059e31f6a