Allow rendering engine to look up for macros from differents sources

This will allow to provide macros registered from the wiki. The main reason for the need of a macro source aggregator component versus Macro.ROLE component lookup is that in virtual mode registering dynamically macros as plexus component would mean sharing those macros accross all wikis, which we do not want.

An idea would be to have a MacroManager component that aggregates MacroSource implementations, default MacroSource being Macro component lookup (as does the current MacroFactory).