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).
The interaction between this class and GtkCellAreaBox could
use some optimization, maybe the cells and their sizes should
be returned as a list and iterated over at the same time as
requesting sizes instead of the hash table approach currently
taken, however the code is clean this way for now.
