Don't use qmlRegisterType with QT-5.14
Summary:
Building Plymouth-kcm-5.17.3 against Qt 5.14.0_beta3 fails to build.
I don't have much experience with QT/KDE programming but these
changes are sufficient to fix the build error on my end.
BUG: 414679
Reviewers: jriddell, Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: zzag, davidedmundson, asturmlechner, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25624