Added most of the request code for GtkCellAreaBox
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).
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