[User Switcher] Port settings window to QQC2+Kirigami FormLayout and modernize UI
Summary:
- Port to QQC2+Kirigami FormLayout style
- Adjust some strings to sound a bit more natural
Test Plan:
Everything was tested and still works.
Reviewers: VDG, Plasma, filipf, broulik
Reviewed By: VDG, Plasma, filipf, broulik
Subscribers: broulik, filipf, plasma-devel
Tags: Plasma
Maniphest Tasks: T10586
Differential Revision: https://phabricator.kde.org/D20899