Widget classes should be added to wrapper rather than widget element

[Marek Lewandowski]

as @Reinmar writes:

Currently widget classes are applied to widget elements. This makes it impossible to achieve various results - e.g. ​​http://stackoverflow.com/questions/29704019/how-to-floatleft-a-container-with-an-image-having-a-relative-width

We wanted to keep classes on elements because we thought that it allows to create the same stylesheets for the content inside editor and outside. Unfortunately, due to the wrappers and CSS's limited capabilities it makes it impossible to achieve some pretty standard results.

Note, that it will not be that uncommon to move classes to the wrapper because e.g. image2 already moves alignment classes there. It's a pity that we didn't learn from that fact earlier ;/.

So all the methods like ​addClass should add to the wrapper rather than to the widget's element.

This is part of #13200 issue.