Multi-wire panel: stable first wire, mixed fields, locked text, one reload

Review of #1205:

- The scene gives no order for its selection, so the "first" wire whose
  values the panel shows could change between two selections of the
  same wires. The wires are now sorted top to bottom, then left to right.
- A text field the selected wires do not agree on (function, section,
  cable...) is shown blank with the hint "Plusieurs valeurs". Typing any
  value there, including the first wire's own, now differs from what was
  shown, so it reaches every wire: 1.5 mm² on a 1.5 and a 2.5 wire works.
- The conductor text is locked while several wires are selected: the same
  number on all of them would make duplicates. "Apply to all conductors
  of this potential" with one wire selected still sets the number on the
  whole potential.
- One edit of N wires emitted N signals, each reloading the panel. They
  are now gathered into one reload, which matters since a reload reads
  every selected wire to find the mixed fields. Selections and targets
  are de-duplicated with sets, and a wire reached through an earlier
  wire's potential is not walked again. An edit that changes nothing
  returns before any potential is walked.

The rules move to sources/conductormultiedit.h, templates over the wire
type, so tst_conductormultiedit can check them against a fake wire:
order, targets with and without potentials, one undo step and its redo
for three wires, and the mixed fields. tst_conductorapplychanges now
gives the other wire the opposite value in every boolean field, so
copying one by mistake fails the test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmake/qet_compilation_vars.cmake

@@ -188,6 +188,7 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/conductorprofile.h
   ${QET_DIR}/sources/conductorproperties.cpp
   ${QET_DIR}/sources/conductorproperties.h
+  ${QET_DIR}/sources/conductormultiedit.h
   ${QET_DIR}/sources/conductorsegment.cpp
   ${QET_DIR}/sources/conductorsegment.h
   ${QET_DIR}/sources/conductorsegmentprofile.h