RFR: 8297554: Remove Scene.KeyHandler [v3]

[Michael Strauß]

> The `Scene.KeyHandler` class doesn't seem to have a clear purpose, mixing focus handling with event propagation. Since #852, `KeyHandler.setFocusVisible` is also called from mouse and touch event handlers, which makes the purpose of the class even less pronounced.
> 
> Moving the focus-related functionality next to the other focus functions in the `Scene` class makes it easier to work with the code in the future.
> 
> With the focus-related functions gone, `KeyHandler` only contains a single, small method that is called from `Scene.processKeyEvent`. For simplicity, this code can be rolled into `Scene.processKeyEvent` and the now-empty `KeyHandler` class can be removed.

Michael Strauß has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Merge branch 'master' into fixes/keyhandler-refactor
 - review changes
 - Removed KeyHandler

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Changes: https://git.openjdk.org/jfx/pull/962/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=962&range=02
  Stats: 190 lines in 1 file changed: 84 ins; 89 del; 17 mod
  Patch: https://git.openjdk.org/jfx/pull/962.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/962/head:pull/962

PR: https://git.openjdk.org/jfx/pull/962