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

{F6813478}

{F6813479}

{F6813480}

Reviewers: #vdg, #plasma, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

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