Improve compatibility with other multiselect widgets

[cubiclesoft]

I just switched Admin Pack (https://github.com/cubiclesoft/admin-pack-with-extras) to use your widget instead of the original multiselect widget upon which this widget is based.

First off, thank you for providing a logical mechanism to easily swap the left and right sides - I always found the layout of the original widget to be really awkward.

The one thing I have to be careful of is not mixing your widget with this one:

https://github.com/ehynds/jquery-ui-multiselect-widget

Admin Pack offers three different multiselect widgets for different reasons. Your widget is useful for selecting and ordering fields for generating reports in CSV format, Eric Hynds' widget is useful for small multiselect lists, and select2 is great for managing "tags". Due to naming conflicts, this widget cannot be used at the same time as Eric Hynds' widget on a page (indirectly raises a Javascript error). It would be nice if there were a no-conflict option for this widget.

[yanickrochon]

There wasn't that many plugins back then with that name! But, yeah, I recognize that there might be a need for this feature. I'll take a look at it today.