With this new approach at request and allocate time, the average child size
is used to determine a good guess at how many columns will fit the box
width; afterwards extra columns are appended and checked to fit.
Then the row heights are calculated based on height-for-width of each
child in the row which now may have individual widths.
Made an enum GtkWrapBoxPacking for the expand/fill horizontal/vertical
boolean options... changed xpadding/ypadding to be horizontal-padding
and vertical-padding for a more consistent api and better readablility.
