Fix wrong argument being passed into maximize component creation
It's supposed to refer to the attached property on QQC2.Overlay, not QQC2.ApplicationWindow
Prevents this runtime error:
qrc:/org/kde/neochat/qml/RoomPage.qml:322:5: QML Component: Unsuitable arguments passed to createObject(). The first argument should be a QObject* or null, and the second argument should be a JavaScript object or a QVariantMap