PRs for consideration after v5.3.6 release

[Jermolene]

As usual, we will collect here the PRs that would have been merged if it weren't for the v5.3.6 release freeze. These PRs should be considered for the next release (apart from quick fix releases).

• Add tc-test-case class to testcase-widget #8744

[Leilei332]

I hope these can be included in the next major release:

• Introduce new MediaQueryTrackerMechanism, and use it for dark mode #8555
• feat: i18n framework based on extracting translations from plugin's /languages/ to /language/ #8435
• Feat: allow link to section mark #7744

[Jermolene]

Thanks @Leilei332 also adding:

• Migrate some old macros to new syntax #8768

[Jermolene]

And also

• Add DiffTextWidget colors to some palettes #8661

[Jermolene]

And

• Extend rendered-plain-text template to support more languages. #8780

[Jermolene]

Adding:

• fix: allow overwrite plugin that don't have plugin-priority #8800

[Jermolene]

I've now merged the straightforward PRs:

• fix: allow overwrite plugin that don't have plugin-priority #8800
• Extend rendered-plain-text template to support more languages. #8780
• Add DiffTextWidget colors to some palettes #8661
• Migrate some old macros to new syntax #8768
• Add tc-test-case class to testcase-widget #8744

The remaining PRs need further investigation: #8555, #8435, #7744