Once a document has been Saved As, don't prompt to save the document upon closing

[Lewis Rosenthal]

PDF form fill-in. Data populated in form. Document | Save As... New name given to copy of document. Exit Lucide. Prompt appears to save the current document.

There are a few reasons for this. The most basic is that when we save a document as another document, we should then switch to that new document for editing (logically). Unfortunately, there does not yet exist a simple Save option (only Save As...), so the only opportunity we get to save the original document is upon exit. See #273 for related behavior.

[Lewis Rosenthal]

We're still not switching to the Saved As file (1.4.1 GA), so:

1. Open a new form to fill in.
2. Document | Save as... and provide new name for "working" copy.
3. Titlebar still reflects original document name.
4. Enter data, save, exit.

Result: original document, not saved as copy, has been edited.

So, this issue is half-fixed. Perhaps the "switch to saved as copy" should be a separate ticket.