[Digital clock plasmoid] Calendar settings page: port to QQC2 & Kirigami and improve layout

Summary:
BUG: 400663
FIXED-IN: 5.15.0

Test Plan:
Before: {F6455075}

After: {F6459462}

(label alignment being a bit off is unrelated to this patch)

Everything still works.

Reviewers: #plasma, #vdg, mart, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: alexde, mvourlakos, abetts, zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17354