RFR: JDK-8304959: Public API in javafx.css.Match should not return private API class PseudoClassState [v2]

[John Hendrikx]

> The class `PseudoClassState` is private API, but was exposed erroneously in the CSS API. Instead, `Set<PseudoClass>` should have been used. This PR corrects this.

John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:

  Ensure Set PseudoClass and StyleClass are immutable in CSS API
  
  Updated documentation, fixed warnings and removed some unnecessary code

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1070/files
  - new: https://git.openjdk.org/jfx/pull/1070/files/dffc6f47..9bbe9259

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1070&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1070&range=00-01

  Stats: 62 lines in 4 files changed: 16 ins; 13 del; 33 mod
  Patch: https://git.openjdk.org/jfx/pull/1070.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1070/head:pull/1070

PR: https://git.openjdk.org/jfx/pull/1070