kcm/workspaceoptions: Add test field for double click setting

Kai Uwe Broulikrequested to merge
work/kbroulik/doubleclck-test into master

A millisecond number is just an abstract concept, let the user test out what it actually means and whether it's sufficiently long enough for them.

Salvaged from !2168 (closed) without the slider

Screenshot_20250324_172414

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