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