Option is on by default as before.
Clock hides in unison with the rest of the UI when unchecked.
Question is what about the mouse cursor? I think it should just always hide when idle (no config option) but there was a Qt bug where `MouseArea` wouldn't apply the `cursorShape` which was since fixed: https://bugreports.qt.io/browse/QTBUG-41045
So we could resurrect D13207 now but without hacks :)