Just always tell the title and cell widgets to
clip their children to the right size. Otherwise
we risk things getting out of sync and unintended
overdraw.
Fixes: #3179
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.
