GUI: Saving of a project shall not be done already at project creation

Initiated by Joachim:

When trying out some little thing with the BornAgain GUI, I am obliged to first choose a project name, and something is written to disk. This I find inconvenient and patronizing. We should follow the example of typical office programs: After launching the program, you either load external data, or create a new simulation. Directory and filename for external storage are only selected if you decide you want to save something.

Original issue: Github#1174