A layout manager instance makes only sense if it's owned by a
container. For this reason, it should have a floating reference
instead of a full reference on construction; this allows constructing
Boxes like:
box = clutter_box_new (clutter_fixed_layout_new ());
without leaking the layout manager instance.
The LayoutManager class is an abstract proxy for the size requesition
and size allocation process in ClutterActor.
A ClutterLayoutManager sub-class must implement get_preferred_width(),
get_preferred_height() and allocate(); a ClutterContainer using the
LayoutManager API will then proxy the corresponding Actor virtual
functions to the LayoutManager instance. This allows having a generic
"blank" ClutterActor sub-class, implementing the ClutterContainer
interface, which leaves only the layout management implementation to
the application developers.
