Design code cleanup

[JPry]

Changes proposed in this Pull Request:

1. Some general code cleanup, specifically:
   • Updating how the meta prefix is handled
   • Ensuring the meta prefix, which is filterable, always uses the potentially filtered value
   • A bit of defensive coding against possible errors
2. Adds the missing Device type section to the source parameters. The simplest solution I could find to facilitate this was the usage of the mobiledetect/mobiledetectlib library, but I'm open to changing this if there's a better solution.
3. Added a JavaScript-powered Details expansion

Checks:

• [x] Does your code follow the WordPress coding standards?

Changelog entry

[JPry]

@rawdreeg regarding your previous comment, I said:

That's a good question. I'm not sure, as we haven't really had many styling needs yet.

This isn't too complex quite yet, but I think I might need to eat my words a bit here 😅