Move arrow to front when hovering over arrow

[fbrckman]

Note
Simply editing the zIndex will not be sufficient. The relationship( arrow)s are drawn on a separate Konva layer. The zIndex only causes changes in the shape within its own layer, and does not change the order in comparison to shapes of other layers.

If we want to achieve that the arrow is drawn over all other shapes, we would have to create a new layer (on top of all the other layers) and draw the relationship in this new layer while hovering over the arrow.