https://github.com/user-attachments/assets/9aba75ff-4190-4a85-89ed-d3f3aa53d3b0
Among other things this adds a new `EmbedderMsg::ShowSelectElementMenu`
to tell the embedder to display a select popup at the given location.
This is a draft because some small style adjustments need to be made:
* the select element should always have the width of the largest option
* the border should be part of the shadow tree
Apart from that, it's mostly ready for review.
<details><summary>HTML for demo video</summary>
```html
<html>
<body>
<select id="c" name="choice">
<option value="first">First Value</option>
<option value="second">Second Value</option>
<option value="third">Third Value</option>
</select>
</body>
</html>
```
</details>
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by
`[X]` when the step is complete, and replace `___` with appropriate
data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] Part of https://github.com/servo/servo/issues/3551
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because ___
<!-- Also, please make sure that "Allow edits from maintainers" checkbox
is checked, so that we can help you if you get stuck somewhere along the
way.-->
<!-- Pull requests that do not address these steps are welcome, but they
will require additional verification as part of the review process. -->
---------
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
