Cb in tab staff exports to MusicXML with playback an octave too low

Windows 7, GIT commit: 4554f23

1) create score for guitar with standard & tab staves
2) enter a Cb into the standard staff
3) export to XML
4) load the XML file
5) click the two representations of the Cb

Result: the playback of the Cb in the tab staff is an octave lower than the playback in the standard staff (you can hear this hitting Play as well, but clicking seems an easier way to hear the problem)

Notes:

I am not 100% this is an actual bug, but it seems to be. See also #22131: diatonic transposition of Cb & Cbb is off by an octave - the same basic type calculation is being used here. lasconic & I found this bug hunting through the code looking for other places where there seemed to be an attempt to calculate octave and step from pitch and tpc. So the same sort of fix would seems to apply. It's possible the tabpitch2xml could invoke the same function I fixed for the previous bug, or that something usable to both functions could be factored out.