Add option to pan ScrollView by dragging

[mgunyho]

Ref #2235

ScrollView has an internal Flickable, with interactive: false. This interactive could be exposed via a public property on the ScrollView (could be called e.g. draggable or pannable), so that the ScrollView can also be panned by dragging the contents if desired. Use case: image viewer that can be panned both with the mouse and with the scroll bars.

A workaround is

scroll := ScrollView {
    HorizontalLayout {
        Rectangle { // this would be the image or whatever you want to pan
            TouchArea {
                moved => {
                    scroll.viewport-x += self.mouse-x - self.pressed-x;
                    scroll.viewport-y += self.mouse-y - self.pressed-y;
                }
            }
        }
    }
}

This looks a bit confusing (at least to me), but it works because moving the viewport also moves the TouchArea, until mouse-x - pressed-x becomes zero. (This simple version doesn't respect the boundaries of the ScrollView, it's possible to pan the inner Rectangle too far.)