Using java.io.File.separator instead of a slash character in se.digiplant.res.api.Res.scala

It is recommended to use java.io.File.separator instead of a single slash character ('/'), because slash is operating system dependent.
However, Microsoft Windows does the trick to replace slashes to backslashes in the background, using slashes is still a bad practice since java (and scala) developers' aim is to write platform independent code where it is possible!