Port to CMake from qmake, and split decoder into subdirectory

Qt 5 has had to be dropped as CMakeLists.txt uses qt_* specific commands
which are implemented in Qt 6 onwards.

cmake --install is only working on Linux at the moment; it installs the
executable and the bundled example tti files into /usr/local/share/doc

The teletext decoder with its document storage has been moved to a
subdirectory with the intention of making it possible to use in other
projects, but some further separation of editing stuff will be needed.

src/qteletextmaker/pagecomposelinksdockwidget.h

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2020-2024 Gavin MacGregor
+ *
+ * This file is part of QTeletextMaker.
+ *
+ * QTeletextMaker 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 3 of the License, or
+ * (at your option) any later version.
+ *
+ * QTeletextMaker 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 QTeletextMaker.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef PAGECOMPOSELINKSDOCKWIDGET_H
+#define PAGECOMPOSELINKSDOCKWIDGET_H
+
+#include <QCheckBox>
+#include <QComboBox>
+#include <QDockWidget>
+#include <QLineEdit>
+#include <QRegularExpressionValidator>
+#include <QString>
+
+#include "mainwidget.h"
+
+class PageComposeLinksDockWidget : public QDockWidget
+{
+	Q_OBJECT
+
+public:
+	PageComposeLinksDockWidget(TeletextWidget *parent);
+	void updateWidgets();
+
+private:
+	void setComposeLinkPageNumber(int linkNumber, const QString &newPageNumberString);
+	void setComposeLinkSubPageNumbers(int linkNumber, const QString &newSubPagesString);
+
+	TeletextWidget *m_parentMainWidget;
+	QCheckBox *m_composeLinkLevelCheckbox[4][2]; // For links 0-3
+	QComboBox *m_composeLinkFunctionComboBox[4]; // For links 4-7; remember to subtract 4!
+	QLineEdit *m_composeLinkPageNumberLineEdit[8], *m_composeLinkSubPageNumbersLineEdit[8];
+
+	QRegularExpressionValidator *m_pageNumberValidator;
+};
+
+#endif