RFR: 8306021: Add event handler management to EventTarget [v5]

[Michael Strauß]

> This PR adds the following methods to the `EventTarget` interface:
> 1. `addEventHandler`
> 2. `removeEventHandler`
> 3. `addEventFilter`
> 4. `removeEventFilter`

Michael Strauß has updated the pull request incrementally with two additional commits since the last revision:

 - added tests
 - Code review changes

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1090/files
  - new: https://git.openjdk.org/jfx/pull/1090/files/2557fb2f..f2236946

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1090&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1090&range=03-04

  Stats: 147 lines in 8 files changed: 119 ins; 20 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/1090.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1090/head:pull/1090

PR: https://git.openjdk.org/jfx/pull/1090