kickoff: Use Item as searchField property type
This works around a Qt bug where assigning the SearchField from the Header fails because of a type mismatch:
"Cannot assign QObject* to TextField_QMLTYPE_8*"
PlasmaExtras.SearchField is a PlasmaExtras.ActionTextField is a QQC2.TextField, so this should work. But for some reason it sees the searchField in [1] as plain QObject instead of what it really is.
I haven’t been able to reproduce it in a minimal example. I assume it’s got to do with declarative type registration, qmltc, and/or the way Plasma creates the full representation vs the main QML.