qrc file generator should allow to change or remove qrc file prefix

Qt.core module can generate .qrc file from artifacts tagged with qt.core.resource_data. The resulting file will have name "_qbs_auto" + product.name + ".qrc". This is not good for Q_INIT_RESOURCE macro which is usually used with a plain library name. However the current implementation in QBS requires to init resources with a magic prefix (which is not documented and could be changed in the future).