RFR: 8345188: Support tree-structural pseudo-classes [v8]

[Andy Goryachev]

On Wed, 8 Jan 2025 19:24:33 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> The CSS Selectors specification defines the `:root` pseudo-class that matches root nodes:
>> https://www.w3.org/TR/selectors-4/#the-root-pseudo
>> 
>> JavaFX uses the non-standard `.root` style class for the same purpose. We should also support the `:root` pseudo-class for increased compatibility of JavaFX CSS with the web specification.
>> 
>> Additionally, we should also support the following child-indexed pseudo-classes:
>> `:first-child`
>> `:last-child`
>> `:only-child`
>> `:nth-child()` with arguments `even` and `odd`
>> 
>> The `An+B [of S]?` microsyntax for `:nth-child()` is not supported, as this would require major changes in JavaFX CSS.
>
> Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:
> 
>   move pseudo-class changes to end of onChanged

Anecdata: I did not see a noticeable performance difference on startup between this PR and the master branch with 48 children with the monkey tester:
![Screenshot 2025-01-08 at 12 22 46](https://github.com/user-attachments/assets/db60c108-ea25-431d-88c7-7d7e8c799703)

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PR Comment: https://git.openjdk.org/jfx/pull/1652#issuecomment-2578586183