RFR: 8297554: Remove Scene.KeyHandler

[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.

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Commit messages:
 - Removed KeyHandler

Changes: https://git.openjdk.org/jfx/pull/962/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=962&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297554
  Stats: 195 lines in 1 file changed: 84 ins; 89 del; 22 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