[effects/cube] Change type of the rotationDuration property to int

Summary:
Currently, type of the rotationDuration is std::chrono::milliseconds.
std::chrono::milliseconds is an unregistered datatype so we can't really
use it with Q_PROPERTY.

Test Plan: Ran qdbus org.kde.KWin /KWin supportInformation.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13839