RFR: 8306021: Add event handler management to EventTarget

[Michael Strauß]

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

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

Commit messages:
 - remove final modifier
 - remove final modifier
 - documentation changes
 - add final modifiers in Dialog class
 - revert some changes
 - doc changes
 - Promote addEventHandler/addEventFilter to EventTarget

Changes: https://git.openjdk.org/jfx/pull/1090/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1090&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306021
  Stats: 584 lines in 13 files changed: 166 ins; 355 del; 63 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