Do not use fixed steps when scrolling on battery icon
Summary:
This makes it possible to have some more fine-grained control over
the screen brightness with scroll devices that support it, like
touchpads or trackpoints.
Test Plan: Tested with touchpad, trackpoint and scroll wheel on mouse
Reviewers: Plasma, broulik
Reviewed By: Plasma, broulik
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D5025