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).