combineWidgets loses class of widgets

[dmurdoch]

htmlwidgets creates a class for the div holding a widget that contains the name of the widget, and some other stuff. combineWidgets just uses class cw-widget, ignoring the original class.

This causes trouble for rgl, because it provides some style info in a CSS file that depends on the class attribute for the div.

I have a patch on dmurdoch/manipulateWidget in the keepclass branch, but it also includes the previous patch. I don't know how to submit a PR that only includes the new stuff.

[bthieurmel]

Thanks. Merge your PR in next_release.