RFR: 8297554: Remove Scene.KeyHandler [v2]

[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 incrementally with one additional commit since the last revision:

  review changes

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Changes:
  - all: https://git.openjdk.org/jfx/pull/962/files
  - new: https://git.openjdk.org/jfx/pull/962/files/14c66129..cb576505

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=962&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=962&range=00-01

  Stats: 12 lines in 1 file changed: 0 ins; 0 del; 12 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