Keep track of when user chooses 'Keep Changes in Editor' so only prompted once

This change is to fix an issue I noticed while researching #5432:

1. Create a new file `foo.html`
2. Type `abc` into it. __Don't save it!__
3. Right click on the file in the tree and Delete it
4. When prompted, choose Keep Changes in Editor
5. Switch to another app and back to Brackets

Results:
You are prompted again if you want to Keep Changes in Editor for this file.

This change keeps track of that response so that you are only prompted once.

Not sure who's best to review this one. Maybe @gruehle or @peterflynn ?