[kcm] Don't transform button to show rotated icon

Summary:
Rotating a button leads to tooltips being broken, it will lead to
shadows being broken on a theme that uses shadows and is generally not
great.

This patch sets the contentItem explicitly and rotates that

BUG: 412092

Reviewers: Plasma, ngraham, romangg

Reviewed By: Plasma, ngraham, romangg

Subscribers: romangg, mart, ngraham, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24488