D7558: kcms/input: Allow WheelScrollLines = 0; some code cleanup

[Marco Martin]

mart added a reviewer: VDG.

REPOSITORY
  R119 Plasma Desktop

REVISION DETAIL
  https://phabricator.kde.org/D7558

To: valeriymalov, #plasma, #vdg
Cc: mart, plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, 
jensreuterberg, abetts, sebas, apol, lukas