new field's responsibility POC: EditableURLWidget allow to edit Bookmark.path in two separated fields, displaying unquoted values which are requoted on form post processing

refactor field's value retreiving from the widget (eg 'display value' concept): * kill field's display_value method, since getting value specified in a previous post or in req.form should be the widget responsibility (the widget nows what inputs to display, not the field) * new typed_value / format_value on widgets, mainly to ease overriding