KCapacityBar: set QStyle::State_Horizontal state

Summary:
QStyleOptionProgressBarV2 has been deprecated in favor of QStyleOptionProgressBar.
Beside that, ::orientation member has also been deprecated in favor of
checking the QStyle::State_Horizontal flag.

We need to set the QStyle::State_Horizontal state so widget styles which
don't use QStyleOptionProgressBarV2 can properly style KCapacityBar.

See http://doc-snapshots.qt.io/qt5-5.11/qstyleoptionprogressbar-obsolete.html

Reviewers: cfeck, elvisangelaccio

Reviewed By: cfeck

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11566