Add support for mensur within staffDef

[martha-thomae]

Mensuration can be encoded using staffDef's attributes, but they can also be encoded using mensur as a child of staffDef.

[JRegimbal]

Is there functionality in mensur that isn't in the staffDef attributes?

[martha-thomae]

@JRegimbal, the functionality is the same regarding the encoding of the mensuration (you use similar attributes in both). But <mensur> allows you to encode a @facs attribute to point to the zone where the mensuration sign appears. This cannot be done with <staffDef>. Even if it could be done, lots of information can be provided in <staffDef> (including, key signature, clef, and mensuration), so one would not be sure what this @facs attribute in <staffDef> is referring to.