xkbcli-compile-compose: Fix string result escaping
Currently the result string is not escaped and may produce invalid
results.
Fixed by introducing an ad-hoc escape function and relative tests.
xkbcli: Promote compose to xkbcli-compile-compose
Previously this tool was only used for internal testing and thus
not installed. But it is useful for debugging, much like
xkbcli-compile-keymap.