cxf-fediz IDP on top of Spring Web Flow

This pull request suggests changing the way to implement an IDP basing on Spring Web Flow instead of filters chain as currently.
The goal is to improve readability and flexibility, notably reducing the IDP java code base.

This pull request is, as requested by Fediz team, stricly scoped to existing features in apache/cxf fediz trunk and then introduce no new feature (such authentication by form or support of remote/requestor IDP)