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/CMakeLists.txt

@@ -0,0 +1,7 @@
+file (GLOB SOURCES *.cpp)
+
+add_library(qteletextdecoder ${SOURCES} teletextfonts.qrc)
+
+target_include_directories(qteletextdecoder PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+target_link_libraries(qteletextdecoder Qt::Widgets)