This way, we no longer prescribe the use of either GtkListItem or
GtkListItemWidget.
This means we can use it in other places, such as for custom section
header objects or with my Canvas ideas.
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.
It's a GtkListItemWidget subclass that tracks the column it belongs to
and allows the column to track it.
We also use this subclass to implement sizing support so columns share
the same size and get resized in sync.
