Get Screen-Coordinates of a QuickControl

As @BaCaRoZzo mentioned, use the mapToItem()/mapFromItem() functions:

import QtQuick 2.0
import QtQuick.Window 2.0
import QtQuick.Controls 1.0

Window {
    id: window
    width: 400
    height: 400
    visible: true

    Button {
        id: button
        text: "Button"
        x: 100
        y: 100

        readonly property point windowPos: button.mapToItem(null, 0, 0)
        readonly property point globalPos: Qt.point(windowPos.x + window.x, windowPos.y + window.y)
    }

    Column {
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.bottom: parent.bottom

        Text {
            text: "Button position relative to window: x=" + button.windowPos.x + " y=" + button.windowPos.y
        }

        Text {
            text: "Button position relative to screen: x=" + button.globalPos.x + " y=" + button.globalPos.y
        }
    }
}

As mentioned in the documentation for mapToItem():

Maps the point (x, y) or rect (x, y, width, height), which is in this item's coordinate system, to item's coordinate system, and returns a point or rect matching the mapped coordinate.

If item is a null value, this maps the point or rect to the coordinate system of the root QML view.

That gives us windowPos. To get the position of the control relative to the screen itself, we just add the x and y position of the window.

After a chat with OP, it's clear that he wants to do this in C++. The same principles apply, and in C++ we have more convenient access to the window:

class Control : public QQuickItem
{
    Q_OBJECT
public:
    Control() {}
    ~Control() {}

public slots:
    void printGlobalPos() {
        qDebug() << mapToItem(Q_NULLPTR, QPointF(0, 0)) + window()->position();
    }
};

Register the type:

qmlRegisterType<Control>("Types", 1, 0, "Control");

Use it in QML:

import QtQuick 2.0
import QtQuick.Window 2.0

import Types 1.0

Window {
    id: window
    width: 400
    height: 400
    visible: true

    Control {
        id: button
        x: 100
        y: 100
        width: 100
        height: 40

        MouseArea {
            anchors.fill: parent
            onClicked: button.printGlobalPos()
        }

        Rectangle {
            anchors.fill: parent
            color: "transparent"
            border.color: "darkorange"
        }
    }
}