LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Libraries/LibGUI/GApplication.cpp

@@ -48,7 +48,7 @@ GApplication::GApplication(int argc, char** argv)
     (void)argv;
     ASSERT(!s_the);
     s_the = this;
-    m_event_loop = make<CEventLoop>();
+    m_event_loop = make<Core::EventLoop>();
     GWindowServerConnection::the();
     if (argc > 0)
         m_invoked_as = argv[0];