Store (x, y) of last motion event.  From
gtk_tree_view_adjustment_changed(), call prelight_or_select() so that
the prelight is recalculated.  We do the same from
gtk_tree_view_size_allocate() for the case that clicking on an expander
shows new rows that resize the column(s) left of the expander.  This
means that the expander is moved horizontally, in such a case the
prelight also has to be reconsidered.