RFR: JDK-8304959: Public API in javafx.css.Match should not return private API class PseudoClassState [v8]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 11 additional commits since the last revision:

 - Merge branch 'openjdk:master' into feature/remove-private-api-in-css
 - Override hashCode with a comment explaining why
 - Fix style issues
 - Restore removed public method
 - Fix PseudoClassTest
 - Add missing null check
 - Fix tests, don't call addAll with null
 - Remove unused methods
 - Fix build by pulling in partial fix for BitSet class
 - Ensure Set PseudoClass and StyleClass are immutable in CSS API
   
   Updated documentation, fixed warnings and removed some unnecessary code
 - ... and 1 more: https://git.openjdk.org/jfx/compare/d22cf2aa...80ff9992

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1070/files
  - new: https://git.openjdk.org/jfx/pull/1070/files/270d0cbd..80ff9992

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1070&range=07
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1070&range=06-07

  Stats: 16879 lines in 239 files changed: 13777 ins; 2251 del; 851 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