RFR: 8351038: ConcurrentModificationException in EventType constructor [v3]

[Andy Goryachev]

> - synchronized `EventType::register()` method
> - simplified internal constructor which is only used for `EventType.ROOT`
> 
> There should negligent impact on performance when `EventTypes` are created in the FX Application Thread.
> 
> There might be a distant potential for a deadlock if the application wraps code that creates `EventTypes` in a block synchronized on a different object.

Andy Goryachev 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 six additional commits since the last revision:

 - formatting
 - Merge remote-tracking branch 'origin/master' into 8351038.event.type
 - ignored
 - Merge remote-tracking branch 'origin/master' into 8351038.event.type
 - test
 - synchronized

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1729/files
  - new: https://git.openjdk.org/jfx/pull/1729/files/552a645d..f44d3b7a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1729&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1729&range=01-02

  Stats: 415 lines in 10 files changed: 391 ins; 14 del; 10 mod
  Patch: https://git.openjdk.org/jfx/pull/1729.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1729/head:pull/1729

PR: https://git.openjdk.org/jfx/pull/1729