Filenames in XML cannot always use backslashes

XML configuration files can refer to other XML files with e.g. agents, policies, and obstacles. The paths to these other files can be both relative and absolute.

Currently, these subfile references need to use forward slashes as folder separators. The function crowdSimulator.cpp::getFolder() (for extracting the folder part of a filename) assumes this. We should make this more flexible, so that other separators are also allowed.