RFR: 8345188: Set :root pseudo-class on root node

[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.

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Commit messages:
 - Set :root pseudo-class on sub-scene root node
 - Set :root pseudo-class on root node

Changes: https://git.openjdk.org/jfx/pull/1652/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1652&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345188
  Stats: 62 lines in 5 files changed: 55 ins; 4 del; 3 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