nls: add native language support

- adds labwc.pot and po files for de, es, it and sv
- added notes in NEWS.md and CONTRIBUTING.md
- addresses #269
- conditional upon `msgfmt` being installed
- can be disabled at build time

include/labwc.h

@@ -48,6 +48,13 @@
 #include "config/keybind.h"
 #include "config/rcxml.h"
 #include "ssd.h"
+#if HAVE_NLS
+#include <libintl.h>
+#include <locale.h>
+#define _ gettext
+#else
+#define _(s) (s)
+#endif
 
 #define XCURSOR_DEFAULT "left_ptr"
 #define XCURSOR_SIZE 24