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/qteletextdecoder/pagex26base.h

@@ -0,0 +1,46 @@
+/*
+ * 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 PAGEX26BASE_H
+#define PAGEX26BASE_H
+
+#include <QByteArray>
+#include <QList>
+
+#include "pagebase.h"
+#include "x26triplets.h"
+
+class PageX26Base : public PageBase //: public QObject
+{
+	//Q_OBJECT
+
+public:
+	X26TripletList *enhancements() { return &m_enhancements; };
+	virtual int maxEnhancements() const =0;
+
+protected:
+	QByteArray packetFromEnhancementList(int packetNumber) const;
+	void setEnhancementListFromPacket(int packetNumber, QByteArray packetContents);
+	bool packetFromEnhancementListNeeded(int n) const { return ((m_enhancements.size()+12) / 13) > n; };
+
+	X26TripletList m_enhancements;
+	const X26Triplet m_paddingX26Triplet { 41, 0x1e, 0 };
+};
+
+#endif