This composite el resolver will be used at the top level resolver for faces
and jsp (the one we add with JspApplicationContext.addELResolver(javax.el.ELResolver).
It keeps track of its scope to let the variable resolver VariableResolverImpl
know in which scope it is executed. This is
necessarry to call either the faces or the jsp resolver head.
This implementation does nothing if there is no actual faces context. This is necessarry since we registered our
resolvers into the jsp engine. Therefore we have to make sure that jsp only pages where no faces context is available
are still working