GTK3 prep: refactor the layout calculation in Columns.

Previously, columns_size_request and columns_size_allocate would each
loop over all the widgets doing computations for both width and
height. Now I've separated out the width parts from the height parts,
and moved both out into four new functions, so that the top-level
columns_size_request and columns_size_allocate are just wrappers that
call the new functions and plumb size and position information between
them and GTK.

Actual functionality should be unchanged by this patch.

unix/gtkcols.h

@@ -45,6 +45,7 @@ struct ColumnsChild_tag {
     /* Otherwise, this entry represents a change in the column setup. */
     gint ncols;
     gint *percentages;
+    gint x, y, w, h;           /* used during an individual size computation */
 };
 
 GType columns_get_type(void);