Cover auto-numbering counter changes with undo/redo

Placing an auto-numbered element or conductor advances a shared
NumerotationContext counter (QETProject::addConductorAutoNum/
addElementAutoNum) as a side effect that sat entirely outside the undo
stack. Undoing the placement removed the visible number but left the
counter advanced, so every undo of an auto-numbered placement silently
burned a number, with no way to get it back short of a manual reset.

Adds SetAutoNumContextCommand, a small QUndoCommand storing the old/new
NumerotationContext and calling the matching add*AutoNum() setter on
undo()/redo() -- the same shape QPropertyUndoCommand already uses next
to it in ConductorAutoNumerotation::applyText().

Wires it into the two conductor call sites (the static newProperties(),
and numerateNewConductor(), both in ConductorAutoNumerotation) and the
element call site (Element::setUpFormula(), called from
DiagramEventAddElement::addElement() when a new element is dropped onto
a diagram). setUpFormula() now takes an optional parent QUndoCommand;
addElement() calls it before pushing its own undo_object so the counter
change lands in the same undo macro as the element's placement -- one
Ctrl+Z reverts both together, instead of leaving the counter adrift.

The project-properties config dialog's own add*AutoNum() calls (editing
the numbering rule itself, not a side effect of placing something) are
deliberately left untouched, as are the load-time folio-sequential
bookkeeping calls in Diagram::loadElmtFolioSeq()/loadCndFolioSeq() and
the bulk folio-renumbering passes in QETProject -- none of those run as
part of an undoable user gesture.

Implements the scope proposed in discussion #608.

sources/qetgraphicsitem/element.h

@@ -35,6 +35,7 @@ class Terminal;
 class Conductor;
 class DynamicElementTextItem;
 class ElementTextItemGroup;
+class QUndoCommand;
 
 /**
 	This is the base class for electrical elements.
@@ -142,7 +143,7 @@ class Element : public QetGraphicsItem
 		{return m_autoNum_seq;}
 		autonum::sequentialNumbers& rSequenceStruct()
 		{return m_autoNum_seq;}
-		void setUpFormula(bool code_letter = true);
+		void setUpFormula(bool code_letter = true, QUndoCommand *parent_undo = nullptr);
 		void setPrefix(QString);
 		QString getPrefix() const;
 		void freezeLabel(bool freeze);