Reapply "Feature: Terminal potential grouping"

This reverts commit 1793dcb9a8.

sources/properties/terminaldata.cpp

@@ -129,6 +129,9 @@ QDomElement TerminalData::toXml(QDomDocument &xml_document) const
 			xml_element.setAttribute("label_color", m_label_color.name());
 	}
 
+	if (!m_potential.isEmpty())
+		xml_element.setAttribute("potential", m_potential);
+
 	// Save master label override settings
 	if (m_use_master_label) {
 		xml_element.setAttribute("use_master_label", "true");
@@ -207,6 +210,8 @@ bool TerminalData::fromXml (const QDomElement &xml_element)
 			m_label_color = QColor(color_str);
 	}
 
+	m_potential = xml_element.attribute("potential");
+
 	// Read master label override settings
 	m_use_master_label = (xml_element.attribute("use_master_label") == QLatin1String("true"));
 	m_master_label_index = xml_element.attribute("master_label_index", "0").toInt();

sources/properties/terminaldata.h

@@ -103,6 +103,18 @@ class TerminalData : public PropertiesInterface
 		*/
 		QString m_name;
 
+		/**
+			@brief m_potential
+			An optional, symbol-author-chosen grouping identifier shared by
+			terminals that belong to the same physical terminal within
+			a multi-terminal block. When potential_isolating is enabled
+			on the parent element, terminals sharing a non-empty,
+			matching m_potential stay linked to each other; terminals
+			with different (or empty) values become isolated from each
+			other. Empty by default.
+		*/
+		QString m_potential;
+
 		/**
 			@brief m_pos
 			Position of the terminal. The second point is calculated