In the instrument wizard, when a staff is changed to standard type (from a previous different type), the standard clef of the istrument is lost.
Changes:
- Added a member variable to `StaffListItem` storing the default standard clef for the instrument.
- Modified `instruments.xml' to remove <clef> tags from tablature instrument definitions: all these instruments are inited from a standard-staff main entry, recording the default standard clef; this clef is used to init the `StaffListItem::_defaultClef` member variable.
NOTE: Percussion instruments are not covered (an number of percussion instruments seem to have old-style definitions in `instruments.xml`); conversion from percussion to standard staff may yield unexpected results.
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