Summary:
In this patch, the settings window is modernized in the following ways:
- Adopt Kirigami+FormLayout style
- Port away from QQC2
- Make the tab switcher UI take up less horizontal space, so the buttons don't get cut off at the default window size

Test Plan: {F6780579}

Reviewers: #plasma, #vdg, filipf, hein

Reviewed By: #plasma, #vdg, filipf, hein

Subscribers: hein, filipf, GB_2, broulik, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20612