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/GFileSystemModel.h

@@ -85,7 +85,7 @@ public:
         bool has_traversed { false };
 
         int m_watch_fd { -1 };
-        RefPtr<CNotifier> m_notifier;
+        RefPtr<Core::Notifier> m_notifier;
 
         GModelIndex index(const GFileSystemModel&, int column) const;
         void traverse_if_needed(const GFileSystemModel&);