KDAB
Support QML Attached properties
I found that support for QML Attached properties is a bit lacking. While most attached property bags appear as child-QObjects in the object tree and can thus be inspected from there, I don't think that having the actual item as the parent item is required by QML so we cannot really rely on that. Also, I think real support inside the QML inspector would be much nicer. I have done some digging, and it seems like this information should be available from the engine.
My ideal solution would entail having any the attached properties appear in the properties inspector view just like grouped properties, only using the attaching QML type name (like "Layout", "Component", "ListView" etc.) as the entry that can then be folded out to reveal the available attached properties, their bindings, etc. They should also appear in Bindings as well as in Signals. Of course, only attached properties and signals for which there actually exist an instance of the underlying attached type; not creating them.
As a technical note, I found that the (internal) functions to retrieve the attached objects have changed (deprecated) in 5.14, so I guess we need different versions of this for Qt 5 and Qt 6.
