Acknowledgement—Work at the Institute of Applied Physics (Darmstadt University of Technology) is financially supported within the Sonderforschungsbereich 185 Nichtlineare Dynamik of the Deutsche Forschungsgemeinschaft. Work at the Institute of Physics (Belgrade) is supported by the Project 01M07 of the Ministry of Science and Technology of the Republic of Serbia. We would like to thank Markus Münkel and Andreas Stepken for valuable and interesting discussions;