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.cpp

@@ -0,0 +1,113 @@
+/*
+	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/>.
+*/
+#include "customelementinfopartwidget.h"
+
+#include "../diagramcontext.h"
+#include "../qeticons.h"
+
+#include <QGridLayout>
+#include <QLineEdit>
+#include <QToolButton>
+
+/**
+	@brief CustomElementInfoPartWidget::CustomElementInfoPartWidget
+	Constructor
+	@param key initial key name (empty for a freshly added row)
+	@param value initial value
+	@param parent parent widget
+*/
+CustomElementInfoPartWidget::CustomElementInfoPartWidget(
+		const QString &key,
+		const QString &value,
+		QWidget *parent) :
+	QWidget(parent),
+	m_key_edit(new QLineEdit(key, this)),
+	m_value_edit(new QLineEdit(value, this)),
+	m_remove_button(new QToolButton(this))
+{
+	m_key_edit->setPlaceholderText(tr("nom_de_la_propriete"));
+	m_key_edit->setToolTip(tr("Lettres minuscules, chiffres, tiret et underscore uniquement"));
+	m_value_edit->setClearButtonEnabled(true);
+
+	m_remove_button->setIcon(QET::Icons::Remove);
+	m_remove_button->setToolTip(tr("Supprimer cette propriété"));
+	m_remove_button->setAutoRaise(true);
+
+	auto *layout = new QGridLayout(this);
+	layout->setContentsMargins(0, 2, 0, 2);
+	layout->setVerticalSpacing(2);
+	layout->setHorizontalSpacing(0);
+	layout->addWidget(m_key_edit,   0, 0);
+	layout->addWidget(m_value_edit, 1, 0);
+	layout->addWidget(m_remove_button, 0, 1, 2, 1);
+
+	connect(m_key_edit, &QLineEdit::textChanged, this, &CustomElementInfoPartWidget::validateKey);
+	connect(m_key_edit, &QLineEdit::textChanged, this, &CustomElementInfoPartWidget::changed);
+	connect(m_value_edit, &QLineEdit::textChanged, this, &CustomElementInfoPartWidget::changed);
+	connect(m_remove_button, &QToolButton::clicked, this, [this]() {
+		emit removeRequested(this);
+	});
+
+	setFocusProxy(m_key_edit);
+	validateKey();
+}
+
+CustomElementInfoPartWidget::~CustomElementInfoPartWidget()
+{
+}
+
+/**
+	@return the key name currently typed in this row
+*/
+QString CustomElementInfoPartWidget::key() const
+{
+	return m_key_edit->text().trimmed();
+}
+
+/**
+	@return the value currently typed in this row
+*/
+QString CustomElementInfoPartWidget::value() const
+{
+	return m_value_edit->text();
+}
+
+/**
+	@return true if the typed key is non-empty and matches
+	DiagramContext::isKeyAcceptable()
+*/
+bool CustomElementInfoPartWidget::hasValidKey() const
+{
+	const QString k = key();
+	return !k.isEmpty() && DiagramContext::isKeyAcceptable(k);
+}
+
+/**
+	@brief CustomElementInfoPartWidget::validateKey
+	Flag the key field when it doesn't match the accepted format,
+	instead of silently dropping it later.
+*/
+void CustomElementInfoPartWidget::validateKey()
+{
+	const QString k = key();
+	if (k.isEmpty() || DiagramContext::isKeyAcceptable(k)) {
+		m_key_edit->setStyleSheet(QString());
+	} else {
+		m_key_edit->setStyleSheet(QStringLiteral("border: 1px solid red;"));
+	}
+}