[Diagram Editor] Provide "Save As..." file browser so user can save the diagram to a specific filepath

[mxgrey]

The current Download button doesn't provide any option besides saving the diagram to a file named diagram.json in the user's Download folder. This is creating significant spam in my download folder where I'm getting

• diagram.json
• diagram (1).json
• diagram (2).json
• ...

piling up. Then I need to figure out which of those files is the one most recently created, and then manually move it to where I actually want it saved, and then manually rename it. This is especially troublesome if I'm iterating on a diagram and want to periodically save it to update it.

It would be a much better user experience if the user can set a filepath and then the file always gets exported there for the rest of the session until the filepath is explicitly changed. It would also be good if we retain the filepath information when a user uploads a diagram, so we can automatically save to that filepath when exporting.