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

Test Plan:
{F6787481}

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