RFR: 8345188: Set :root pseudo-class on root node [v2]

[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.

Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:

  update cssref

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1652/files
  - new: https://git.openjdk.org/jfx/pull/1652/files/1299f0b2..bdb33c2f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1652&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1652&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 1 del; 7 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