ScrollBar: Make it work as a horizontal scroll bar as well

Summary:
It was only taking into account the height so always working as vertical.
This tracks the orientation property

BUG: 390351

Test Plan: Now case explained in the bug report works just fine, couldn't see any regressions.

Reviewers: #kirigami, mart, ngraham

Reviewed By: #kirigami, mart, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21131