Add StContainer, which implements the ClutterContainer interface based
on the container methods in st-private and make the existing containers
subclass it.
https://bugzilla.gnome.org/show_bug.cgi?id=613907
Ideally we'd be able to override _paint, but given that we can't
at the moment, this method gives a way to implement containers
which don't happen to paint all of their children.
https://bugzilla.gnome.org/show_bug.cgi?id=603522
Subclass ClutterGroup (to avoid having to implement all of dispose,
raise, lower, add, etc.), and have it proxy the allocation requests
out into signals. We have to group up the two out parameters
into a struct unfortunately.
Included example code in the C file source for now.
