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/GTextDocument.h

@@ -146,7 +146,7 @@ private:
     bool m_client_notifications_enabled { true };
 
     GUndoStack m_undo_stack;
-    RefPtr<CTimer> m_undo_timer;
+    RefPtr<Core::Timer> m_undo_timer;
 };
 
 class GTextDocumentLine {