refactor(kcm): move rotation buttons in own module
Summary:
Simple refactor to make the code more readable when later on auto rotation
functionality is added.
Test Plan: KCM starts up.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26028