Add class based on widget type to the widget wrapper

[Marek Lewandowski]

As @Reinmar writes:

Inability to write rules matching certain widget wrapper + widget class

Once classes will be moved to wrappers it would be good if it was possible to write rules like:

.image.centered { ... }
.embed.centered { ... }

As I experienced recently centered image's wrapper may need very different styles than that of an embed widget. E.g. image's wrapper must have text-align: center. This affects embed widgets which may be unwanted if we only want to use margin-left/right: auto.

Hence, it will be useful if widget wrapper had classes based on widgets names - e.g. "widget-image", "widget-embed", etc.