st-container: Remove ClutterContainer implementation

Now that ClutterActor has a ClutterContainer implementation, we
can start removing StContainer. To help make this a bit more
understandable, instead of converting everything at once, make
StContainer a compatible API wrapper around the ClutterActor
implementation, and then we'll remove those wrappers in later
commits.

https://bugzilla.gnome.org/show_bug.cgi?id=670034