Lock Screen: Restore option to show clock only during unlock prompt

Originally, the clock was always shown on the logout prompt. However, this was changed, and now the clock is either always there or never there.

This adds a third option restoring original behavior and allowing a proper screen saver use case again (e.g. no clock when idle but the clock when the logout prompt is shown)

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Needs plasma-workspace!5298 (merged)

Could probably be optimized to set visible: false on the Clock and DropShadow when hidden but I think we didn’t do this originally either