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.
This uses a custom GtkColumnViewTitle widget. So far that widget is
pretty boring, but that will change once we added
resizing, reordering, dnd, sorting, hiding/showing of columns or
whatever UIs we want.
The ColumnView now allocates column widths first and then the individual
rows use the new layout manager which looks at the column allocations to
allocate their children.
