Add secondary editor window on launch; rename/refactor primary VC; extract attachments

- AppDelegate: open SecondaryWindowController at launch, position next to main,
  keep strong refs to controllers, set title "TextEdit Empty".
- New SecondaryTextEditViewController: empty STTextView with completions, line
  numbers, toolbar actions for wrap/invisibles.
- Rename ViewController -> PrimaryTextEditViewController; extract
  applySampleDecorations; use defaultText; clean up delegate conformance.
- Move NSTextAttachment providers into new TextAttachments.swift.
- Storyboard: update main VC class, add Secondary Window Controller with toolbar,
  link to SecondaryTextEditViewController.
- Xcode project: add new files, remove old ViewController.swift, update sources.