Upgrade to latest OpenJFX version / fix headless UI-tests

[kaklakariada]

When upgrading to the latest OpenJFX version 20, UI-Tests that open a new window (e.g. org.itsallcode.whiterabbit.jfxui.AboutDialogUiTest) started to fail.

The change in OpenJFX occured between versions 20-ea+9 (works) and 20-ea+11 (stopped working).

Relevant changes in OpenJFX sources: openjdk/jfx@20+9...20+11
Possible root cause: openjdk/jfx@5b0d3b5

See also TestFX/TestFX#749, waiting for new release of https://github.com/TestFX/Monocle

[kaklakariada]

Fixed with #265