Optimize ratsnest line endings for zones.

Zones islands have a single anchor in the connectivity system. This means that often when a net has unconnected clusters containing zones the closest anchors will be pads. But this is really counter-intuitive to the user as the pad is clearly connected to the zone, so why does it have a ratsnest line coming off of it?

This MR does a post-process of any ratsnest lines adding additional anchors at reasonable points to any zones involved and setting the ratsnest line source and/or target anchors to the new anchors.

Before the change this board will show ratsnest lines between the three pads (two of which are clearly connected to the zone):

This is the result after the MR: