Add hints alignment x slot

[aadcg]

Description

A follow-up to #3226.

Fixes #3179.
Fixes #3229.

Checklist:

• Git branch state is mergable.
• Changelog is up to date (via a separate commit).
• New dependencies are accounted for.
• Documentation is up to date.
• Compilation and tests ((asdf:test-system :nyxt/gi-gtk))
  • No new compilation warnings.
  • Tests are sufficient.

[MaxGyver83]

I have tested it and it works as expected.

A follow-up to #3236.

I think this is a typo and you wanted to refer to PR #3226.

[aadcg]

@MaxGyver83 I have only rebased your work thus far. I'll add more commits soon and ping you for a review!

Thanks for the typo catch!

[aadcg]

I've refactored the work that @MaxGyver83 started in 1be0228, while retaining the commit.

I believe that it meets the users needs in the most general possible way while maximizing maintainability on the developer's side.

The draft, as it stands, breaks backwards compatibility so it won't make it for the 3-series. I don't think this feature is a priority so that's OK.

Looking for a first round of reviews on this draft to ensure that we're all on the same page. CC @MaxGyver83 @jmercouris. Then I'll polish it and make it ready to be merged.

Screenshots:

[jmercouris]

Looks OK to me. Not sure what the magic number of -100 is, and why a font size of greater than 5 for that condition, but otherwise fine :-)

[aadcg]

Superseded by #3318 since the name of this branch is misleading.