[trunk] Oddity and possible crash with score excerpts

Setup:trunk rev. 4220 both under Win XP (Qt lib 4.7.0) and Ubuntu (Qt lib 4.6.2)

Steps:

1) Open any score with parts, for instance the attached sample score
2) Close it
3) Open any other score with parts

Result:

1) More tabs are created than the ones really needed for the score/parts open in step 3), some of them with title(s) referring to the score open in step 1)
2) Tab titles do not correspond to their contents
3) By clicking on the extra tabs (the ones at the right), the programme stops unexpectedly.