Throughout this project N.M.has been variously funded by INFN; the European Re-search Council under the European Union’s Seventh Framework Program (FP/2007–2013);ERC Grant Agreement n.307286(XD-STRING);and the Italian Ministry of Education,Universities and Research under the Prinproject“Non-Perturbative Aspects of Gauge Theories and Strings”(2015MP2CX4). J.M. is supported by the FPI grant BES-2013-064815 of the Spanish MINECO,and the travel grantEEBB-I-17-12390 of the same institution. He also acknowledges partial support through the Spanish and Regional Government Research Grants FPA2015-63667-P and FC-15-GRUPIN-14-108. D.P.is funded by ERC Grant Agreement n.307286(XD-STRING).