The method in ol.Overlay is not exposed whereas is equivalent
getter getPositioning is visible in API and documentation.
The PR add a line of description to the documentation and expose
the method
Missing CSS was confirmed with issue #680, this commit is to fix it and change
ol-mouse-position class to ol-mouseposition. I choose for the moment the top
right corner to display the coordinates from mouse position control because of
potential conflict with the scaleline control.
Thomas