[MusicXML export] invalid display-step and display-octave for moved rests on tablature staff

The current trunk sometimes generates an invalid MusicXML file when a tablature staff contains a rest that has been moved (i.e. in the .mscx file the rest would have a pos element). In the MusicXML file, display-step and/or display-octave may contain invalid values.

As the rests in a tablature staff are typically hidden anyway, a simple fix is to prevent writing display-step and display-octave for rests on a tablature staff.

See http://musescore.org/en/node/19027 for examples: files test_1_0.mscz and test_2_0.mscz demonstrate the problem.