Update application tutorial to remove O'Closure and use Ojwidgets instead

This is mostly done, I need feedback on how the O'closure section has been changed since i'm not sure if it is understandable.
I need to retest it fully and I think it'll be OK, from my point of view. Feedbacks are more than welcome, I know my english isn't that good and I think the rewritten part is now quite harsh and it could be better. So if you have any suggestions. :-)