Calling gtk_list_box_remove_all() is a no-op with a bound model; after
the introduction of the remove_all() method in 49e56fc7, we were left
with row widgets after the dispose() call chained up.

We could restore the explicit unparenting inside dispose() instead of
calling remove_all(), but since the bound list model is provided by the
user of GtkListBox, it's more appropriate to unbind it in the dispose()
implementation, to avoid any potential reference cycle (especially in
higher level languages that have no explicit reference acquisition).

We clean up the bound model, and its associated state, if any; and then
we remove all the row widgets that are left.