Add "Potentiel" field to the terminal editor (element editor)

New QLineEdit (m_potential_le) between Type and Nom: an optional,
symbol-author-chosen grouping identifier shared by terminals that
belong to the same physical terminal within a multi-terminal block.

Stored as TerminalData::m_potential (new field, persisted as the
"potential" XML attribute, empty by default).
Used by relatedPotentialTerminal() in terminal.cpp: when
potential_isolating is enabled on a Terminal-type element, terminals
sharing a non-empty, matching potential value now stay electrically
linked to each other instead of every terminal in the block being
isolated from every other one.

Tooltip on the field for explanation

sources/editor/graphicspart/partterminal.cpp

@@ -476,6 +476,15 @@ void PartTerminal::setMasterLabelIndex(int index)
 	emit masterLabelIndexChanged();
 }
 
+void PartTerminal::setPotential(const QString &potential)
+{
+	if (d->m_potential == potential) return;
+	prepareGeometryChange();
+	d->m_potential = potential;
+	update();
+	emit potentialChanged();
+}
+
 /**
 	Updates the position of the second point according to the position
 	and orientation of the terminal.