Provide theming functions for common webform components

[algeronwaterschoot]

Not all webform components are rendered equally. Some have extra wrappers, others have extra classes. Components aren't rendered using template files, so it's not immediately clear at first glance how to change their HTML.

We could provide one or several placeholder preprocess/theming functions (as many as we need) in the template, where frontend developers can easily modify the HTML of these components without having to dig through the webform module stack.