[Kicker] Modernize settings window

Summary: Port Kicker's settings window to Kirigami+FormLayout and QQC2.

Test Plan:

All features tested and still work.

Reviewers: VDG, Plasma, hein, GB_2

Reviewed By: VDG, Plasma, hein, GB_2

Subscribers: GB_2, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20743