Build Painter & friends into LibC. Use it in the GUI test app.

Kernel/ProcessGUI.cpp

@@ -111,6 +111,8 @@ int Process::gui$invalidate_window(int window_id)
     if (it == m_windows.end())
         return -EBADWINDOW;
     auto& window = *(*it).value;
+    // FIXME: This should queue up a message that the window server process can read.
+    //        Poking into its data structures is not good.
     WindowManager::the().invalidate(window);
     return 0;
 }