The ClutterActor:depth property has always been a bit of a misnomer:
actors are 2D flat surfaces, so they cannot have "depth"; the property
defines the position on the Z axis.

Another side effect of the :depth property is that it decides the
default paint and allocation order on insertion, and that setting it
will call the ClutterContainer.sort_depth_order() method. This has
proven to be a fairly bad design decision that we strung along from the
0.x days, as it gives a false impression of being able to change the
paint and allocation order simply by changing the position on the Z
axis — something that, in reality, requires depth testing to be enabled
during the paint sequence of an actor's parent.

For 2.0 we need a clean break from the side effects, and a better
defined interface.

ClutterActor:z-position is essentially what ClutterActor:depth is, but
doesn't call into ClutterContainer, and has a more apt name.