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