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/diagramcommands.cpp

@@ -75,6 +75,13 @@ void PasteDiagramCommand::redo()
 	{
 		first_redo = false;
 
+		//make new uuid for every pasted conductor, because old uuid are
+		//the uuid of the copied conductor
+		const QList <Conductor *> all_pasted_conductors = content.conductors();
+		for (Conductor *c : all_pasted_conductors) {
+			c -> newUuid();
+		}
+
 		//this is the first paste, we do some actions for the new element
 		const QList <Element *> elmts_list = content.m_elements;
 		for (Element *e : elmts_list)