[jfx19] RFR: 8291502: Mouse or touch presses on a non-focusable region don't clear the focusVisible flag of the current focus owner

[Michael Strauß]

The `focusVisible` flag is only set on a node when it acquires input focus using a keyboard interaction, and it is cleared by mouse and touch interactions.

It is not necessary for a node to lose input focus in order to lose `focusVisible`. Currently, clicking on a region of the scene that does not contain a focusable node does not clear the `focusVisible` flag.

Detecting clicks or touches that should clear `focusVisible` can be achieved by adding an event filter for `MOUSE_PRESSED` and `TOUCH_PRESSED` to the `Scene`.

There are two additional noteworthy changes with this PR:
1. Adding event filters to `Scene` causes many tests to fail due to a bug in `MouseEventFirer` that was identified in [8253769](https://bugs.openjdk.org/browse/JDK-8253769). Previously, mouse events generated by `MouseEventFirer` skipped a code path that causes test failures due to incorrect coordinates. With the added event filters, the code path for mouse events is slightly different, exposing the incorrect coordinates and causing tests to fail.
This problem can be solved by making the "alternative" `MouseEvent` generation path the default path.

2. The changes around `KeyHandler` might seem to be excessive for the scope of this PR. However, this is mostly just code that was moved to another place (it was moved right next to the rest of the focus-related functionality in `Scene`). `KeyHandler` was a dubious class before (since it didn't seem to have a clear purpose, mixing focus handling with event propagation), but with the need to call `KeyHandler.setFocusVisible` from mouse and touch event handlers, its purpose became even less pronounced.
I think that moving the focus-related functionality next to the other focus functions in the `Scene` class is a safe and straightforward change, and it 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, I decided to roll this code into `Scene.processKeyEvent` and remove the now-empty `KeyHandler` class.

Note: Since this is a follow-up fix for [8268225](https://bugs.openjdk.org/browse/JDK-8268225), I'm targeting this fix for `jfx19`.

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Commit messages:
 - Mouse or touch press clears the focusVisible flag
 - Failing test

Changes: https://git.openjdk.org/jfx/pull/852/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=852&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291502
  Stats: 243 lines in 3 files changed: 142 ins; 85 del; 16 mod
  Patch: https://git.openjdk.org/jfx/pull/852.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/852/head:pull/852

PR: https://git.openjdk.org/jfx/pull/852