app,app/internal/window: [macOS] add app.Window.Close for closing a window

Recently support was added for multiple top-level windows. Add support
for closing those windows.

macOS only; all others stubbed out.

Signed-off-by: Larry Clapp <larry@theclapp.org>

app/internal/window/window.go

@@ -57,6 +57,9 @@ type Driver interface {
 	ReadClipboard()
 	// WriteClipboard requests a clipboard write.
 	WriteClipboard(s string)
+
+	// Close the window.
+	Close()
 }
 
 type windowRendezvous struct {