Insert default clef if system does not have a clef

If the MEI file generated by a previous OMR step does not indicate the clef of a system, insert a default clef.

The goal of having this default clef is rendering all neumes encoded in the MEI file. As discussed, we could the clef of the previous system for all systems > 1 or the clef of the next system for the first system. Since this clef will be a guess, it should be highlighted. After user's corroboration, it should be de-highlighted.