Popup: Missing transform property

It is weird that there's a transformOrigin property for QtQuick.Controls2 Popup but without providing the transform property (the rotation property is missing too, comparing to Item).

That makes it inconvenient (and may be impossible) to apply complex transformations to Popups because we only have the scale property for Popups currently.

As a workaround we could hack into the QQuickPopupItem::transform:

Popup {

    Scale { id: popupScale }

    Component.onCompleted: background.parent.transform = popupScale
}

However I think depending directly on the implementation details is usually not a good idea.