Added save/restore for the last src image name

• Asking `NSUserDefaults.standardUserDefaults` for a given key in `- viewDidLoad`; if the value exists, switches to it; if it doesn’t, defaults to the `_srcPossibilityNames.firstObject` (as before).
• At the end of `- switchToImageNamed:` saves the now-successful imageName back into the defaults.
• That’s it.  Easy!