[FIX] correctly update many2many in listview (web client)

Fixes the issue #1216 (follow the link for more information). The issue
was caused by a hack in list view: the magical suffix __display is used
in render_cell to determine if a many2many field should be updated. This
commit simply makes sure that old many2many fields + __display keys are
cleared.

A better way would be to redesign/refactor the list view to avoid that
hack in the first place.  But this would be a much more complex task.