Don't re-open doc when user didn't save it

Some follow-up to #330...

In that thread way back on July 28, @uranusjr wrote:

> \2. Open a new document. Edit it and quit (without saving). Start the app.
>
> The unsaved document re-appears (still not saved), nothing else.

This is not correct behavior. Here's the flow:
1. Quit MacDown with a document open and never saved.
2. Tell MacDown **not** to save the document when it asks.
3. Re-open MacDown and here's the problem:
- Actual behavior: MacDown reopens the document you told it not to save.
- Desired behavior: MacDown opens with a blank doc.

I don't think any other app I use re-opens an unsaved doc the user explicitly told the app not to save, so this should be changed so that MacDown just throws away that unsaved doc upon quitting.

Reproduce the documented flow in MacDown: quit with an unsaved document, choose not to save, and relaunch. Trace the document-restoration behavior involved in that flow; done means relaunching opens a blank document instead of the discarded unsaved document. The issue names no source file or test, so locating the relevant entry point will require repository research.

Written by the indexing model from the issue text.