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/LibHTML/ResourceLoader.h

@@ -34,7 +34,7 @@ namespace LibProtocol {
 class Client;
 }
 
-class ResourceLoader : public CObject {
+class ResourceLoader : public Core::Object {
     C_OBJECT(ResourceLoader)
 public:
     static ResourceLoader& the();