Show connection lines for nano header points

[nilclass]

When connecting points on the Nano header (e.g. A1, A2, D12, D13, ...) the connection points light up in the correct colors. However, there are no lines shown to the other nodes that are connected, and when hovering the net, these points are not highlighted in any way.

To make it easier to see what's connected to those points, let's draw lines to the other connection points.

Open questions:

• should these lines be visible all the time, or just when hovering the respective net?
• do we draw just one line from each of the points to the closest connected node, or lines to all of the connected nodes in the main section of the board?

[nilclass]

I added basic support for this in a3c952c.

For now the lines are drawn straight from pin to pin, no fancy routing. They are always visible, and get highlighted (line a bit thicker) when the net is highlighted (e.g. when hovering over it in the netlist panel).