[WIP] Added tag for 2.3 compatibility with decorators

CmfMenuBundle 2.0 will use decorators, which is only available since 2.5. This tag will be the BC layer for this feature until we drop 2.3 support, which will be in May 2016.

This PR needs some tests
The core doesn't tag, but a new config option added to the container config. There is no way to easily copy that for 2.3