Add a simple function for checking for membership in the fieldListMap.

git-svn-id: svn://anonsvn.kde.org/home/kde/trunk/kdesupport/taglib@572680 283d02a7-25f6-0310-bc7c-ecb5cbfe19da

ogg/xiphcomment.cpp

@@ -205,6 +205,11 @@ void Ogg::XiphComment::removeField(const String &key, const String &value)
     d->fieldListMap[key].clear();
 }
 
+bool Ogg::XiphComment::contains(const String &key) const
+{
+  return d->fieldListMap.contains(key) && !d->fieldListMap[key].isEmpty();
+}
+
 ByteVector Ogg::XiphComment::render() const
 {
   return render(true);