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/LibConfig/CMakeLists.txt

@@ -0,0 +1,11 @@
+set(SOURCES
+    Client.cpp
+)
+
+set(GENERATED_SOURCES
+    ../../Services/ConfigServer/ConfigServerEndpoint.h
+    ../../Services/ConfigServer/ConfigClientEndpoint.h
+)
+
+serenity_lib(LibConfig config)
+target_link_libraries(LibConfig LibIPC)