add simple toggle to hide/show clock on lock screen

Part of #133 (closed).

This replaces the existing option "Clock: Keep visible when unlocking prompt disappears" with a simple toggle that controls whether the clock is always or never shown on the lock screen. From the discussion when the old option was implemented, ( https://phabricator.kde.org/D24893 ) the use case for the old option was for users who want an unobstructed view of the lock screen background. This use case can still be satisfied by the new option, and perhaps the simplicity of the new option will make it easier to discover (the old description in the UI was rather awkward). The difference with the new option is that it does not matter whether the user is currently typing their password. With the old option, the clock would still be shown when the user moved the mouse or started typing, which didn't give users a choice to not see a clock at all.

Fixes https://bugs.kde.org/show_bug.cgi?id=472587

See also plasma-workspace!4499 (merged)