Give Conductor its own persisted uuid

Slice 1 of discussion #503 (from-to wiring list built on projectDataBase).

Conductor is the one item type on a diagram without a stable identity
of its own -- Element and Diagram both have a uuid, Conductor didn't.
This is the prerequisite the wiring-list tables need: a conductor
table keyed by uuid, the same way the existing element table is keyed
by Element::uuid().

- Conductor gets a QUuid m_uuid, generated in the constructor, with
  uuid()/newUuid() accessors mirroring Element's exact pattern.
- toXml()/fromXml() read/write a "uuid" attribute the same way
  Element already does, including the same generate-on-missing
  fallback (QUuid(e.attribute("uuid", QUuid::createUuid().toString())))
  for projects saved before this change.
- PasteDiagramCommand::redo() calls newUuid() on every pasted
  conductor (content.conductors(), all three categories), mirroring
  the existing per-element newUuid() call right above it -- otherwise
  copy-paste would duplicate a conductor's uuid.

Backward compatibility: Conductor::valideXml() doesn't require the
"uuid" attribute, so old files parse unchanged. Verified by opening a
genuinely pre-uuid project (examples/industrial.qet, 150 folios, 671
conductors, legacy integer terminal1/terminal2 references with no
uuid attribute at all) -- loads and renders correctly, gets uuids
assigned on load, and those uuids are stable across a second
load/save cycle (byte-identical uuid values). Verified paste
separately: copying a selection with conductors and pasting produces
distinct new uuids for every pasted conductor, none colliding with
the originals or each other.

sources/qetgraphicsitem/conductor.h

@@ -21,6 +21,7 @@
 #include "../conductorproperties.h"
 
 #include <QGraphicsPathItem>
+#include <QUuid>
 
 class ConductorProfile;
 class ConductorSegmentProfile;
@@ -77,6 +78,8 @@ class Conductor : public QGraphicsObject
 		int type() const override { return Type; }
 		Diagram *diagram() const;
 		ConductorTextItem *textItem() const;
+		QUuid uuid() const {return m_uuid;}
+		void newUuid() {m_uuid = QUuid::createUuid();}	//create new uuid for this conductor
 		void updatePath(const QRectF & = QRectF());
 
 		//This method do nothing, it's only made to be used with Q_PROPERTY
@@ -205,6 +208,7 @@ class Conductor : public QGraphicsObject
 		Highlight must_highlight_;
 		bool m_valid;
 		bool m_freeze_label = false;
+		QUuid m_uuid;
 
 			/// QPen et QBrush objects used to draw conductors
 		static QPen conductor_pen;