On Mac: Fix App Quitting & Focus on window when save prompt is shown on quit

[andrewalisa]

These fixes are only when you run the Electron Desktop App on Mac.

I noticed these two issues from the README:

1. Quit doesn't exit app completely, only closes the window if it's open
2. Quit doesn't show/focus window when save changes prompt is shown on Mac

I have now fixed these two issues.

Now when I press Cmd + q the app quits. If there is a save prompt, it will focus on that window.