Replace warps-slider checkbox with radio buttons

Summary:
This was suggested by Thomas in
https://git.reviewboard.kde.org/r/128475/ but it probably got lost.

The "Left mouse warps scroll bar" checkbox is very hard to understand, so
we replace it with a buttongroup with two radio buttons that explain
what actually happens in either case.

Reviewers: broulik, colomar

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4563