Let a script style a conductor

setConductorProperty()/conductorProperty() now also take the properties
that control how a conductor looks, under the names the .qet file uses:

  style        normal, dashed or dashdotted
  bicolor      true/false, with color2 as the second colour
  dash-size    positive integer
  condsize     positive number (line width)
  numsize      positive integer (text size)
  displaytext  true/false

Values are validated rather than stored: a boolean other than
true/false, a non-positive size, an unparseable colour or a line style the
file format cannot express is refused, because ConductorProperties would
write the latter back as a solid line and silently lose it. The change is
still applied to the whole potential.

Verified: all eight set, read back, written to the file in its own form
(style as "line-style: dashed;", displaytext as 0) and read back
again after a reload. Four invalid values decline.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sources/scripting/qetscriptapi.h

@@ -99,7 +99,10 @@ class QetShapeItem;
 	  where present, identical across every instance of that element -- so
 	  it does not distinguish one placed coil's A1 from another's.
 	- @b Conductor properties and @b cross-references: set a conductor's
-	  number, formula, colour or section, and link a master to a slave or
+	  number, formula, colour or section (and its look: style normal/
+	  dashed/dashdotted, two-colour mode and second colour, dash size,
+	  line width, text size, whether its number is shown -- all under the
+	  names the .qet file uses for them), and link a master to a slave or
 	  one report to another. Both follow the application's own rules rather
 	  than writing the field: a conductor property is applied to every
 	  conductor of the same electrical potential, which is what the GUI and