applet: Improve code quality
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Put {braces} around if-statement
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Improve code style and property references
The Item's parent property is just an Item object, it doesn't necessarily have a header property. An ID reference, on the other hand, is both typed and unambiguous.
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Explicitly specify arguments in signal handlers
Relying on engine automatically injecting them is non-obvious, and deprecated in Qt 6 for a reason.