Fontless headings

This makes it a lot easier to e.g style the fast ft headings, where the basic style is that of a heading, but possibly wrapping additional content which shouldn't be styled with the heading's fonts.

Now there are additional classes and placeholders available:

* `o-ft-typography-headingN`
* `o-ft-typography-headingN-layout`
* `o-ft-typography-headingN-text`

So it's then possible to do

```html
<div class="o-ft-typography-heading1-layout">
    <span class="o-ft-typography-heading1-text">Heading 1</span>
    <span>With alternatively styled sub-content</span>
</div>
```

Does this seem ok? @dan-searle @danskinner