[Calendar] Port settings window to QQC2+Kirigami FormLayout and modernize UI

Summary:
- Port to QQC2+Kirigami FormLayout style
- Adjust some strings to be more natural
- Use radio buttons instead of a two-item combobox

Test Plan:
Everything works.

{F6793395}

Reviewers: #vdg, #plasma, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20829