Improve handling of relative paths with backslashes

Consider this snippet:

<parent>
  <relativePath>..\pom.xml</relativePath>
</parent>

Note in particular the backslash used in the path. While this path will be properly resolved on a Windows box, it will fail to resolve the local parent on a Unix/Mac box (as the backslash is not a separator but escape character here). This applies to other paths in the POM as well.

For platform-independent behavior, we should either encourage the user to use forward slashes (by means of validation warnings/errors) or have Maven convert the paths internally.