Updated plugin baking task folder list

It's more convenient to remove unneeded folders.
Propagates the various possibilities of plugins.

I was thinking about adding ``Locale``, too.
Any opinions about that?

I'm currently setting up a boilerplate repo for building new plugins, by going a bit beyond of what the plugin bake task provides (WIP!):
https://github.com/ravage84/cakephp-plugin-skel/

It's similar to https://github.com/FriendsOfCake/app-template

But the scope of my repo is beyond of what the core needs to provide.