Since we set the proxy value when the slider changes and set the slider
value on proxy property changes, we run into a cycle.
Before commit 3d3dca4aa this was addressed by not notifying on all slider
changes, but only in reaction to direct user action. Given that since the
splitting out of the BarLevel class those events are handled in a subclass,
that approach is at least unconvential and fairly fragile.
Instead, make the brightness indicator ignore any changes to the slider it
initiated itself.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1500
21e14bd46f