[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
do we not need to use the same tag as symfony does, to be compatible? otherwise we need to use cmf_core.decorator everywhere, even on newer symfony versions.

i would have thought we do an exact copy, and only load it when the original symfony class does not exist.