Add two new cross-platform methods to PathUtilities

- PathUtilities.OpenDirectoryInExplorer opens the directory in
  Explorer or the default file manager.
- PathUtilities.OpenFileInApplication opens a file in the default
  application (like double clicking on the file in explorer).

Change-Id: I199897ca5fe2c3fb78b13a7f71a5e21c7ca36457
No, it's not shorter. However, the calling code might be clearer when you try to make the code cross-platform compatible. Having the Process.Start(file) in the code always makes me wonder if this works on all platforms whereas if I have the explicit method it is probably a little clearer. But I agree, it's debatable if this method is needed and does much good.

I can remove it if you think it would be better without.