implement background scrolling prevention without moving the body

[alex3683]

The current implementation to prevent from scrolling the background sets the body to fixed and translates its position to the current scroll position. This gives the visual effect, that the scroll position is kept as the user left it, but scrolling of the body isn't possible anymore.
One drawback of this approach is that now also all other layers are translated with the body element. Hence, e.g. a message display, that is at the topmost location on the z-axis, is translated out of view and thus isn't visible anymore.
Therefore an alternative approach of background scrolling prevention needs to be implemented.

[alex3683]

Fixed on master (v1.2.0) and release-1.1.x (v1.1.2).