LibCore: Introduce Directory

Core::Directory represents an existing directory on the system, and it
holds an actual file descriptor so that the user can be sure the
directory stays in existence.

Userland/Libraries/LibCore/CMakeLists.txt

@@ -5,6 +5,7 @@ set(SOURCES
     ConfigFile.cpp
     Command.cpp
     DateTime.cpp
+    Directory.cpp
     DirIterator.cpp
     ElapsedTimer.cpp
     Event.cpp