[anchor] get source/targetAnchorCorrection from view

[JanKoehnlein]

When an edge has an arrow head, we have to adjust the anchor point with respect to the stroke width and angle of the arrow head. Currently we store this correction values in the SModel (SRoutableElement#sourceAnchorCorrectionand #targetAnchorCorrection).

We should consider fetching these values from the view, as that is where the arrow head is defined.