fix #12557 untitled score doesn't save (review needed)

[AntonioBL]

This pull request tries to fix the fact that untitled scores are not saved (or, actually, saved in the folder where mscore starts, without mscz extension and without warnings for overwriting). It mimics the use of the GUI save button. For scores with parts, it first switches to the main score and then saves.

[AntonioBL]

Please review the method I used for switching to the relevant tab. I am not sure if I used the right method.
In particular, I found that I have to use these consecutive instructions:
setCurrentView(0,i);
tab1->setExcerpt(0);
setCurrentView(0,i);
without the last line of these three, the subtabs of the window for a score with parts are not properly updated.
I am not sure what would happen if saveFile() is not able, for whatever reason, to save the file: the checkdirty function as in this pull request would not return true.