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
It's not obvious to me that calling this is shorter or clearer than just calling Process.Start.