Add class with sib-core element's name

Actually, it's hard to point native widgets with CSS because of the way they're generated, if I want to make some style for every inputs - but only those from sib-core - then, I need to point every kind of widgets.

I believe that it would makes things easier if every element would have their name as class, allowing to select [class^="solid-form-"] or even [class^="solid-"] with CSS.