Make each shield wire uniform and allow cable.shield color

As the spline shield wires were rendered as 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.

Each shield wire is now rendered equally along the spline sections
and in cable nodes. If cable.shield is true, they are rendered as
thin black wires in the same way as before multi-colors were
introduced. The new feature is that cable.shield is allowed to
contain a two-letter color code to specify a colored shield wire
with black borders.

The shield wire thickness is not increased, even if the cable has
some multi-colored wires that makes all other wires to increase.

This fixes bug #125.

src/wireviz/Harness.py

@@ -230,7 +230,14 @@ 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>'
+                if isinstance(cable.shield, str):
+                    # shield is shown with specified color and black borders
+                    shield_color_hex = wv_colors.get_color_hex(cable.shield)[0]
+                    attributes = f'height="6" bgcolor="{shield_color_hex}" border="2" sides="tb"'
+                else:
+                    # shield is shown as a thin black wire
+                    attributes = f'height="2" bgcolor="#000000" border="0"'
+                html = f'{html}<tr><td colspan="{len(p)}" cellpadding="0" {attributes} port="ws"></td></tr>'
 
             html = f'{html}<tr><td>&nbsp;</td></tr>'  # spacer at the end
 
@@ -248,8 +255,8 @@ def create_graph(self) -> Graph:
                 if isinstance(connection_color.via_port, int):  # check if it's an actual wire and not a shield
                     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']))
+                    # shield is shown with specified color and black borders, or as a thin black wire otherwise
+                    dot.attr('edge', color=':'.join(['#000000', shield_color_hex, '#000000']) if isinstance(cable.shield, str) else '#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'