[quicklaunch] Port settings window to QQC2+Kirigami FormLayout and modernize UI

Summary:
- Port to QQC2+Kirigami FormLayout style
- Use standard import names
- Adjust some strings to sound a bit more natural
- Use Kirigami ActionTextField so that the text field can have a clear button
- Fix a bug preventing the text field from being disabled once enabled

Test Plan:
- All features still work
- Settings are saved and loaded correctly

On the desktop: {F6815666}

In a horizontal panel: {F6815665}

In a vertical panel: {F6815667}

Reviewers: #vdg, #plasma, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D21088