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

[Marius Hanl]

On Mon, 12 Jun 2023 18:17:17 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> 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

Marked as reviewed by mhanl (Committer).

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PR Review: https://git.openjdk.org/jfx/pull/1090#pullrequestreview-1475754096