MusicXML export does not export clefs after removing first measures

A uncommon use case, but it bites down hard when you hit it.

Use case:
* Edit a piece
* Remove some measures, including the first one
* export MusicXML

The resulting MusicXML file does not have clef info. Yet, when saving/opening in native MuseScore format, the clefs are there.

In the export of the "full" (original) file, the clefs are there.

Workaround: replace every clef for every part with another one, and then put the right clef back.
E.g., for the bassoon part in the example, replace the bass clef with a trable clef, and then change back to bass clef.