Port widget settings window to QQC2+Kirigami FormLayout

Summary:
- Port to QQC2+Kirigami FormLayout style
- Use standard import names
- Adjust one string to sound a bit more natural (configuration is implicit and does not need to be mentioned)

Test Plan: {F6817093}

Reviewers: #vdg, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D21104