Implement fs::get_cache_dir

Win32: equal to config dir for now
Linux: respect XDG_CACHE_HOME if specified
OSX: possibly incomplete

Utilities/File.h

@@ -484,6 +484,9 @@ namespace fs
 	// Get configuration directory
 	const std::string& get_config_dir();
 
+	// Get common cache directory
+	const std::string& get_cache_dir();
+
 	// Get data/cache directory for specified prefix and suffix
 	std::string get_data_dir(const std::string& prefix, const std::string& location, const std::string& suffix);