Improve `Directory` API

Jürg Billeterrequested to merge
juerg/vdirectory into master
  • Support (sandboxed) follow_symlinks in FileBasedDirectory.descend(), in line with CasBasedDirectory
  • Add public Directory.exists() method based on improved versions of the internal _exists() methods
  • Add public Directory.open_file() method

This helps plugins being ported to virtual directories.

Part of #1262 (closed), and also #719 (closed).

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