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/ChanViewer/ThreadCatalogModel.h

@@ -63,5 +63,5 @@ private:
 
     String m_board { "g" };
     JsonArray m_catalog;
-    RefPtr<CHttpJob> m_pending_job;
+    RefPtr<Core::HttpJob> m_pending_job;
 };