qml: downcast items from `QObject` to `QQuickItem` in `ExpandGridView.qml`

As their names suggest, they are supposed to be QQuickItem pointer.

Unfortunately when Loader started to gain ability to load QObject in Qt 5:

Since QtQuick 2.0, Loader can load any object type.

They have not changed the name of the property name item to object, which can be confusing. I suppose they re-purposed item as a generic "item", not necessarily QQuickItem. Still, this goes in conflict with Instantiator's object (not named item) (which always had the ability to load QObject).

I have checked this on Qt 6.2.4 and Qt 6.8.2.

Request review @chub.