Allow to pass comments to translators

Comment blocks starting by 'xgettext:' will find there way to vlc.pot and
the .po. You can use it to pass instructions or context to the translators

po/Makevars

@@ -10,7 +10,7 @@ subdir = po
 top_builddir = ..
 
 # These options get passed to xgettext.
-XGETTEXT_OPTIONS = --directory=.. --keyword=_ --keyword=N_ --keyword=_NS --keyword=_ANS --keyword=qtr --keyword=Q_ --language=C++
+XGETTEXT_OPTIONS = --directory=.. --keyword=_ --keyword=N_ --keyword=_NS --keyword=_ANS --keyword=qtr --keyword=Q_ --language=C++ --add-comments=xgettext:
 
 # This is the copyright holder that gets inserted into the header of the
 # $(DOMAIN).pot file.  Set this to the copyright holder of the surrounding