[kcmkwin] Use QtQuick.Controls 2.0 for Label and TextField

Summary:
Using QtQuick.Controls for Label and TextField can result in blurry font rendering for a fractional scaling (e.g. 1,5). There is a work around for QtQuick.Controls 2.x therefore using QtQuick.Controls 2.0 for Label and TextField resolves the problem

BUG: 366451

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: ngraham, zzag, davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D14495