Export segment groups

[PaulHax]

Folks are asking to download segment group images in a format of their choosing.

Suggest we add a download button here:

That pops a modal simlar to the "Save Session" with 2 inputs:

• filename (let filename default to segment group name)
• exported image file format (list of supported formats)

Could have some GUI to set the segment group image format in the session.volview.zip file and utilize the logic in #527. But perphaps a one off download button per segment group scratches the use cases. So configuring format in session.volview.zip is only possible via config.JSON file.

[aylward]

I like it (the button, its location, and the functionality)!