feat: Add keyboard shortcut for Save As

[Travis]

There should be a keyboard shortcut for "Save As..." in the File menu. On macOS, most document-based applications use cmd-shift-S for the shortcut, which currently isn't used by Vensim. I recommend this.

I needed to save a copy of a model and did the expected keyboard shortcut and it didn't work. I would expect it to work as this is standard on macOS.

[Administrator]

I'll try and add it in. Programming for MacOS is proving to be quite a challenge at the moment.