Add user-defined custom properties on elements

ElementInfoWidget's fixed ~40 predefined ELMT_* keys had no way for a
user to add a genuinely new element-info key, even though DiagramContext
already stores/round-trips arbitrary keys generically via toXml()/fromXml().

Adds an "Ajouter une propriété personnalisée" button that appends a
CustomElementInfoPartWidget row (both key and value user-editable,
unlike the fixed ElementInfoPartWidget rows bound to one predefined
key). The typed key is validated live against the existing
DiagramContext::isKeyAcceptable() and flagged with a red border when
it doesn't match, instead of silently dropping it. Any key already
present on the element that isn't one of the predefined/special keys
is re-displayed as a custom row on next selection.

Implements the scope proposed in discussion #611.

sources/ui/customelementinfopartwidget.h

@@ -0,0 +1,61 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef CUSTOMELEMENTINFOPARTWIDGET_H
+#define CUSTOMELEMENTINFOPARTWIDGET_H
+
+#include <QWidget>
+
+class QLineEdit;
+class QToolButton;
+
+/**
+	@brief The CustomElementInfoPartWidget class
+	A single row letting the user define their own element information
+	key/value pair, unlike ElementInfoPartWidget which is bound to one
+	predefined key. The key is validated against
+	DiagramContext::isKeyAcceptable() as the user types.
+*/
+class CustomElementInfoPartWidget : public QWidget
+{
+		Q_OBJECT
+
+	public:
+		explicit CustomElementInfoPartWidget(
+				const QString &key = QString(),
+				const QString &value = QString(),
+				QWidget *parent = nullptr);
+		~CustomElementInfoPartWidget() override;
+
+		QString key() const;
+		QString value() const;
+		bool hasValidKey() const;
+
+	signals:
+		void changed();
+		void removeRequested(CustomElementInfoPartWidget *self);
+
+	private slots:
+		void validateKey();
+
+	private:
+		QLineEdit   *m_key_edit;
+		QLineEdit   *m_value_edit;
+		QToolButton *m_remove_button;
+};
+
+#endif // CUSTOMELEMENTINFOPARTWIDGET_H