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/FileManager/main.cpp

@@ -72,7 +72,7 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    RefPtr<CConfigFile> config = CConfigFile::get_for_app("FileManager");
+    RefPtr<Core::ConfigFile> config = Core::ConfigFile::get_for_app("FileManager");
 
     GApplication app(argc, argv);