[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.

Reviewers: VDG, Plasma, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

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