[System Load Viewer] Port settings window to QQC2+Kirigami FormLayout and modernize UI

Summary:
- Port to QQC2+Kirigami FormLayout style, except for the SpinBox which cannot handle decimal values well in the QQC2 version
- Use standard import names
- Adjust some strings to sound a bit more natural

Test Plan:
- All features still work
- Settings loaded correctly when the window appears and saved correctly

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Reviewers: #vdg, #plasma, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D21056