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/Services/ConfigServer/CMakeLists.txt

@@ -0,0 +1,18 @@
+serenity_component(
+    ConfigServer
+    REQUIRED
+    TARGETS ConfigServer
+)
+
+compile_ipc(ConfigServer.ipc ConfigServerEndpoint.h)
+compile_ipc(ConfigClient.ipc ConfigClientEndpoint.h)
+
+set(SOURCES
+    ClientConnection.cpp
+    main.cpp
+    ConfigServerEndpoint.h
+    ConfigClientEndpoint.h
+)
+
+serenity_bin(ConfigServer)
+target_link_libraries(ConfigServer LibIPC)