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

po/meson.build

@@ -0,0 +1,12 @@
+i18n = import('i18n')
+add_project_arguments('-DGETTEXT_PACKAGE="' + meson.project_name() + '"',
+  '-DLOCALEDIR="' + get_option('prefix') / get_option('localedir') + '"',
+  language:'c')
+i18n.gettext(meson.project_name(),
+  args: ['--directory=' + source_root,
+    '--add-comments=TRANSLATORS',
+    '--no-location',
+    '--keyword=_',
+    '--msgid-bugs=https://github.com/labwc/labwc/issues'],
+    preset: 'glib'
+)