[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:
{F6800326}

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