Make shield wires in cable nodes equal as spline shield wires

As the spline shield wires are rendered as thin tinned wires with
black borders, and the shield wires in cable nodes were rendered as
a single (bottom) border, they didn't fit well together.

The shield wires in cable nodes are now rendered equally as the spline
shield wires. The shield wire color is moved into a common definition.

This fixes bug #125.

src/wireviz/Harness.py

@@ -14,6 +14,10 @@
 import re
 
 
+# shield wires are shown as thin tinned wires
+shield_color_hex = wv_colors.get_color_hex('SN')[0]
+
+
 class Harness:
 
     def __init__(self):
@@ -230,7 +234,7 @@ def create_graph(self) -> Graph:
                 for bla in p:
                     html = html + f'<td>{bla}</td>'
                 html = f'{html}</tr>'
-                html = f'{html}<tr><td colspan="{len(p)}" cellpadding="0" height="6" border="2" sides="b" port="ws"></td></tr>'
+                html = f'{html}<tr><td colspan="{len(p)}" cellpadding="0" height="6" bgcolor="{shield_color_hex}" border="2" sides="tb" port="ws"></td></tr>'
 
             html = f'{html}<tr><td>&nbsp;</td></tr>'  # spacer at the end
 
@@ -249,7 +253,7 @@ def create_graph(self) -> Graph:
                     dot.attr('edge', color=':'.join(['#000000'] + wv_colors.get_color_hex(cable.colors[connection_color.via_port - 1], pad=pad) + ['#000000']))
                 else:  # it's a shield connection
                     # shield is shown as a thin tinned wire
-                    dot.attr('edge', color=':'.join(['#000000', wv_colors.get_color_hex('SN', pad=False)[0], '#000000']))
+                    dot.attr('edge', color=':'.join(['#000000', shield_color_hex, '#000000']))
                 if connection_color.from_port is not None:  # connect to left
                     from_port = f':p{connection_color.from_port}r' if self.connectors[connection_color.from_name].style != 'simple' else ''
                     code_left_1 = f'{connection_color.from_name}{from_port}:e'