Userland: Introduce ConfigServer and LibConfig

ConfigServer is an IPC service that provides access to application
configuration and settings. The idea is to replace all uses of
Core::ConfigFile with IPC requests to ConfigServer.

This first cut of the API is pretty similar to Core::ConfigFile.

The old:

    auto config = Core::ConfigFile::open_for_app("App");
    auto value = config->read_entry("Group", "Key");

The new:

    auto value = Config::read_string("App", "Group", "Key");

ConfigServer uses the ~/.config directory as its backing store
and all the files remain human-editable. :^)

Userland/Libraries/CMakeLists.txt

@@ -4,6 +4,7 @@ add_subdirectory(LibC)
 add_subdirectory(LibCards)
 add_subdirectory(LibChess)
 add_subdirectory(LibCompress)
+add_subdirectory(LibConfig)
 add_subdirectory(LibCore)
 add_subdirectory(LibCoredump)
 add_subdirectory(LibCpp)