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.

Applications/SoundPlayer/PlaybackManager.h

@@ -70,5 +70,5 @@ private:
     RefPtr<ABuffer> m_next_buffer;
     RefPtr<ABuffer> m_current_buffer;
     Vector<RefPtr<ABuffer>> m_buffers;
-    RefPtr<CTimer> m_timer;
+    RefPtr<Core::Timer> m_timer;
 };