RFR: 8345188: Support tree-structural pseudo-classes [v5]

[Michael Strauß]

> 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:

  whitespace error

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1652/files
  - new: https://git.openjdk.org/jfx/pull/1652/files/b52816e3..afb5fa40

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1652&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1652&range=03-04

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1652.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1652/head:pull/1652

PR: https://git.openjdk.org/jfx/pull/1652