This change adds a shadow-tree widget for `<input type=color>` elements.
It also involves some changes to the way layout interacts with the DOM,
because currently all `input` and `textarea` elements are rendered as
plain text and their descendants are ignored. This obviously doesn't
work for `<input type={color, date, range, etc}>`.
<details><summary>HTML used for the screenshot above</summary>
```html
<input type=color>
```
</details>
Testing: I doubt that this affects WPT tests, because the appearance and
behaviour of the widget is almost entirely unspecified.
---------
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
