[MusicXML] Import/export hidden staves

Since MuseScore is able to hide all measures in a system which are empty, it would be logic to be able to persist this feature out to MusicXml.
This can be done, quite simply according to Michael Good (of MusicXml), by adding the following to the first measure to be hidden :

<attributes>
    <staff-details print-object="no" />
</attributes>


with the following when the measures are to be displayed again :

<attributes>
    <staff-details print-object="yes" />
</attributes>


See forum entry "Hide Empty staves - persistence into MusicXml" for the source of this issue.