Adding guitar tab with different tuning

@swabere
This looks like a bug that was resolved for the 6.0 update. Until it's available, changing tuning is described as the part of the answer here:
https://forums.symphonypro.net/discussion/comment/2980#Comment_2980

After choosing a part that contains guitar tab from the Instruments Menu, the Properties menu will provide options to configure the part's tuning. These options are illustrated: