Added the following to GtkCellAreaBox:
- GtkCellAreaBox:spacing property defines spacing between cells
- GtkCellAreaBox:align-cells property defines whether cells should
be aligned with cells in adjacent rows.
- Implementations for get_preferred_width / get_preferred_height
- Implementations for get_preferred_height_for_width and the other
when the box is oriented in the easy way (i.e. height_for_width()
implemented for a vertical box, no virtual allocations done yet).
Added the child list to GtkCellAreaBox, added _pack_start() and
_pack_end() apis to GtkCellAreaBox since they are appropriate there
and implemented GtkCellLayoutIface to override the _pack_start()/end()
methods (since the base GtkCellArea class simply forwards these apis
to the generic ->add() api on the base class).
