Improve visualization relationships

[fbrckman]

• Arrow positioning #163 Start from the constraint instead of the shape.
• Label on relationship #162 Label on the arrow to indicate which kind of relationship is visualized.

Further improvement:

• Single arrow starts at a shape and can split and go into the different objects. At the splitting point, there could be an icon or symbol to indicate the type of relationship (or, and, xone, property).