Clarifying the use of staffDef vs clef

[lpugin]

Currently the musicxml2mei stylesheet generates clef elements for clef changes occurring within a measure and staffDef for the ones occurring at the end of a measure. I would suggest to make it consistent, and this should also be a recommendation for the guidelines. My proposal would be to use clef for all clef changes that occur within the flow of the music, be in within or at the end of a measure (when a clef change at the end of a measure appears before the end of a measure it does not make sense to make it a staffDef after the measure).

staffDef / scoreDef should be used in other cases (including changes with keySig or meterSig). A staffDef / scoreDef with a clef change is still possible and will still make sense but in such a case the clef will be expected to be displayed at the beginning of the next measure.