Fix life-cycle handling of treeview columns

Fix life-cycle handling of treeview columns

gtk_tree_view_remove_column was first removing the column from
its list, then call gtk_tree_view_column_unset_tree_view, which
would then call gtk_container_remove to remove its button from
the treeview. But the treeview remove implementation relied
on the column being still in the list in order to recognize
the button as 'special', so in effect the button was never
properly removed and thus, leaked.

Fix this by callling unset_tree_view before removing the
column from the list.

https://bugzilla.gnome.org/show_bug.cgi?id=724891