Make the DropDownMultiple widget degrade properly.

DropDownMultiple now extends the original SelectMultiple widget and
always renders both widgets. The DropDownMultiple widget is disabled and
hidden by default in the HTML and is enabled by Javascript code, keeping
things working even when Javascript is not available.

As an added advantage, we can now reuse some of the code from
SelectMultiple. In particular, by using the render_options method, we
should now also support more complicated (like grouped) lists of
choices.