BEM Update to Textfield component

Reviewer: @sgomes 

I'm sure I've missed something!
I'm not sure we get much benefit in making "wsk-input" the top level component. Conceptually, it makes more sense to me for a text input to be an element (e.g. wsg-textinput or wsk-textfield), rather than for an input to be one, with text being a sub-component.