MouseArea: positioning mode

Currently there is no easy way to get the client or screen position of the mouse with qml. Sometimes, e.g when adjusting the position of another Window or Tooltip, it is necessary to get the position relative to window or screen origin.

To get the client or screen position right now, you have to manually walk the scene graph and add the relative position of each Item.

I propose a new property for MouseArea, called "posititioning" or similar, with the enum values:
MouseArea.ItemPosition, MouseArea.WindowPosition and MouseArea.ScreenPosition which defaults to the MouseArea.ItemPosition to allow adjusting the origin used for the properties mouseX, mouseY and position.