Feature request: Allow for more flexibility for part creation/editing

I'm not sure if this has already been suggested or in the works, but I'd like to bring up that creating parts is quite rigid if you want to add a staff for cues, or show several instruments in one part.
My understanding is that part creation can only be done from the File menu. Once a custom part is created, you can't add or remove instruments (even though you can press the minus button, it doesn't do anything). So it seems if you want something slightly different, you have to create the part again from scratch.
I propose that the Instruments panel be available (perhaps in a slightly different form) in the Parts so it's easy to add or remove existing instrument parts.
Use cases:
- showing both duet instruments in part of an orchestral piece
- cue staves