[MU4 Issue] Parts can not be saved as individual MuseScore files

[henkdegroot]

Describe the bug In MuseScore 3 it was possible to save a part as a separate/new musescore file. In MuseScore 4 this is no longer possible.

To Reproduce Steps to reproduce the behavior:

1. Open a score with multiple instruments
2. Generate parts
3. Select a part tab
4. Use the Save As option

Expected behavior When a Part tab is selected, only the selected Part should be saved. Alternative, allow this to be done using the Export option.