[qtmozembed] Added private window attribute to QuickMozView

Yes but this exposes an API to QML. Normally boolean properties do not include is-prefix see e.g. active property (calls SetIsActive). Similarly to Item's visible, enabled, and so on.

"window" on the other is trickier as it is easy to associate to QQuickWindow or some kind of windowing mode -- QuickMozView itself is QQuickItem. From QML API point of view this property is not about windowing it's about privacy and that should be emphasized.