Sort programming languages like in the documentation.

Sort programming languages like in the documentation.

* gettext-tools/src/xgettext.c (usage): For the -L, -a, -k, --flag options, sort
the list of programming languages like in the documentation. Add Guile, Rust,
GSettings, that were forgotten earlier.
* gettext-tools/doc/xgettext.texi: Likewise.
* gettext-tools/examples/README: Sort the examples like the programming
languages in the documentation.
* gettext-tools/examples/Makefile.am (EXAMPLESFILES, EXAMPLESDIRS): Likewise.
* gettext-tools/examples/check-examples (func_check_autoclean_all,
func_check_distclean_all, func_check_maintainerclean_all,
func_check_maintainerclean_vpath_all, func_check_dist_all,
func_check_dist_vpath_all, func_check_install_all, func_check_uninstall_all,
func_check_distcheck_all, func_check_all): Likewise.
* gettext-tools/examples/po/Makefile.am (POTFILES, SMALLPOTS, hello-*.pot,
SMALLPOFILES_FOR_lang, $(srcdir)/../hello-*/po/$(LL).po): Likewise.