Allow forward mouse event to wallpaper item

Use childMouseEventFilter on containmentinterface to get mouse events,
and forward them to specified wallpaper item.

This allow wallpaper to get mosue event when desktop layout is "Folder Vew".
It's useful for wallpaper plugin to do something interactive.
Like tracking mouse effect, parallax effect and click response.

Reference:
QQuickWindowPrivate::deliverMouseEvent
QQuickWindowPrivate::deliverHoverEvent

Example:

// main.qml
MouseArea {
    id: mouseArea
    ....
}
Component.onCompleted: {
   wallpaper.acceptedButtons = Qt.LeftButton;
   wallpaper.mousePassTarget = mouseArea;
}