columnviewlayout: Handle for_size propertly

columnviewlayout: Handle for_size propertly

When we are given a for_size as width for the whole
column view, we need to distribute it over the columns
as gtk_column_view_allocate_columns would, in order
to find out which for_size to give to each cell.

This is a bit recursive, but works. Since we are
doing this recursion for every row, we should consider
adding a cache for those distributed widths.