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:

Reviewers: VDG, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

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