Support mirroring of QML anchor layouts

Item anchors are already symmetric (support for both anchors.left and anchors.right), but it should be possible to easily mirror the direction of the defined anchors. See attached patch mirrored_anchors.diff for an example implementation and layoutsamples.zip for an example QML code. See parent task QTBUG-11042 for more info about QtQuick right-to-left locale support.

Item {
    LayoutMirroring.enabled: true
    anchors.left: parent.left // left becomes right
    anchors.leftMargin: 3
}