The BinLayout should store a pointer to the Container that it is
using it as the layout manager.
This allows us to fix the API and drop the additional Container
arguments from set_alignment() and get_alignment().
This also allows us to add a ClutterBinLayout::add() method which
adds an actor and sets the alignment policies without dealing with
variadic arguments functions and GValue (de)marshalling.
Each actor managed by a BinLayout policy should reside inside its
own "layer", with horizontal and vertical alignment. The :x-align
and :y-align properties of the BinLayout are the default alignment
policies, which are copied to each new "layer" when it is created.
The set_alignment() and get_alignment() methods of BinLayout can
be changed to operate on a specific "layer".
The whole machinery uses the new ChildMeta support inside the
LayoutManager base abstract class.
A BinLayout is a simple layout manager that allocates a single cell,
providing alignment on both the horizontal and vertical axis.
If the container associated to the BinLayout has more than one child,
the preferred size returned by the layout manager will be as big as
the maximum of the children preferred sizes; the allocation will be
applied to all children - but it will still depend on each child
preferred size and the BinLayout horizontal and vertical alignment
properties.
The supported alignment properties are:
* center: align the child by centering it
* start: align the child at the top or left border of the layout
* end: align the child at the bottom or right border of the layout
* fill: expand the child to fill the size of the layout
* fixed: let the child position itself
