LibCore: Add Core::Group abstraction for group management :^)

This adds the Core::Group C++ abstraction to ease interaction with the
group entry database, as well as represent the Group entry.

Core::Group abstraction currently contains the following functionality:
- Add a group entry - 'Core::Group::add_group()'

Userland/Libraries/LibCore/CMakeLists.txt

@@ -13,6 +13,7 @@ set(SOURCES
     File.cpp
     FilePermissionsMask.cpp
     GetPassword.cpp
+    Group.cpp
     IODevice.cpp
     LocalServer.cpp
     LocalSocket.cpp