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. It's particularly useful for avoiding having to override hover effects on links, with the detrimental effect that has on specicifity

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