Release: bump VERSION_DEVEL.
PO: update template.
Release: update news.
Merge remote branch 'origin/master' into release/unstable
Release: bump VERSION.
Revert "Remove definition of the "instrumentName" property from Score_performer"

This reverts commit d0e3e48668636f4c66ecdc11f8e297d9d653364a.

See issue 3581 for discussion.