[Digital clock plasmoid] Calendar settings page: port to QQC2 & Kirigami and improve layout
(label alignment being a bit off is unrelated to this patch)
Everything still works.
Reviewed By: Plasma, davidedmundson
Subscribers: alexde, mvourlakos, abetts, zzag, plasma-devel
Differential Revision: https://phabricator.kde.org/D17354