Issues with SI.Distance and MB.World

 * Added inner Modelica.Mechanics.MultiBody.World as required by Modelica.Mechanics.MultiBody.Visualizers.Advanced.Arrow and Modelica.Mechanics.MultiBody.Visualizers.Advanced.DoubleArrow

 * The state variable PlanarMechanics.Joints.Prismatic.s is of type Modelica.SIunits.Distance which means that the minimum length is zero. This is not suitable for simulation with negative elongations. See also [Mo#446](https://trac.modelica.org/Modelica/ticket/446) where there was the same problem.
 * Did some cosmetics.