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.

.gitignore

@@ -1,6 +1,4 @@
-.qmake.stash
-moc_*
-qrc_*
-*.o
 Makefile
-qteletextmaker
+build*/
+cmake-build-*/
+.idea