[Kickoff] Modernize settings window layout

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:

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