fix jQuery UI Multiselect plugin for jQuery UI 1.11.x

The fix are described in
[here](http://www.trirand.com/blog/?page_id=393/bugs/jqgrid-4-5-4-column-chooser-bug#p31105).
The code are base on jQuery UI Multiselect plugin Apr 8, 2013 from
https://github.com/michael/multiselect/