RFR: 8306021: Add event handler management to EventTarget [v3]

[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 one additional commit since the last revision:

  doc changes

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1090/files
  - new: https://git.openjdk.org/jfx/pull/1090/files/7db7f79a..cec4b90b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1090&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1090&range=01-02

  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 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