conform/actor: Add unit for the Container signals

conform/actor: Add unit for the Container signals

ClutterActor should be emitting signals defined on the ClutterContainer
interface, as well as ensuring that manipulating the scene graph is
still possible from within them.

The new unit checks that we're emitting signals, by implementing
something similar to the Bin class available in toolkits like gtk, st,
and mx — i.e. a container that can only hold one child at any given
point.