Deprecate SessionHelper

Current inconsistencies:
- The Session component is deprecated, the helper not.
- The Cookie has not readonly access in the view, whereas the session does.

The way I see it we have two options here:

a) 
Add CookieHelper as READ ONLY wrapper around $this->request->cookie() just as
SessionHelper is one for $this->request->session()
This would allow shorter read-access to those in the view layer.
But they both would be basically just very dumb wrappers.

b) (this PR)
The SessionHelper, just as the component, is not of too much use anymore.
The flash stuff is in the FlashHelper, and as such this helper can probably be removed in favor of

    $this->request->session()

It is just slightly more to type.
But people can still build their own Session and Cookie helpers in userland code if desired.