8345188: Support tree-structural pseudo-classes

[mstr2]

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.

/reviewers 2
/csr

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Issue

• JDK-8345188: Support tree-structural pseudo-classes (Enhancement - P4)

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@mstr2
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 2 (with at least 1 Reviewer, 1 Author).

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@mstr2 please create a CSR request for issue JDK-8345188 with the correct fix version. This pull request cannot be integrated until the CSR request is approved.

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Webrevs

• 04: Full - Incremental (afb5fa40)
• 03: Full - Incremental (b52816e3)
• 02: Full - Incremental (58363439)
• 01: Full - Incremental (bdb33c2f)
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[kevinrushforth]

This will require a change to cssref.html to list the :root pseudo-class in the list of pseudo-classes for Parent.

Additionally, changes will likely be needed to the two places where we say that JavaFX doesn't support structural pseudo-classes, here and here, since :root is a structural pseudo-class.

[mstr2]

I've expanded the scope of this enhancement to also include several child-indexed pseudo-classes, which are also structural pseudo-classes.

I specifically didn't include the :empty structual pseudo-class (matching a Parent with no children), as this pseudo-class is alrady being used by ComboBoxListViewSkin. Perhaps we should deprecate the use of standard pseudo-classes in skins?