overlay - allow position from right

Added "right" as a setting for the position of the Overlay.

I needed the overlay to stick to the right side of the window for all screensizes and on horizontal scrolling.

commit-message:

    works like the "left"-setting but:
    * has no 'center' (unneeded - use left:'center' instead
    * defaults to false (unused), is only used if a pixel or % value is set```