Fix EZP-22597: Use composition on extended FragmentRenderers

https://jira.ez.no/browse/EZP-22597

Currently fragment renderers are extended by eZ Publish by simple inheritance, to add the current SiteAccess as an attribute of the final URI.
Problem is that it prevents one to use Symfony > 2.3 as [there have been a BC break](https://github.com/symfony/symfony/commit/70f3399f7ede50fce570b64c2fa937dbeb040845#diff-433fb9f2dba246ecc181b23a6091404a), leading to fatal errors when using Symfony 2.4.
Idea is to use composition instead of inheritance to get rid of these errors.