Add Storekit reviewRequest()

[vdhamer]

StoreKit allows you to bug users up to three times per year for a rating or review (if they haven't already provided one?). So this is mainly about delaying calls to requestReview() until the app estimates it has been used enough that the user might be willing to provide feedback.

So this is about maintaining counters (in UserDefaults) that keep track of in-session or cross-session statistics. Current idea is to collect experience 'points' whenever the user activates a feature. And to trigger requestReview() at say multiples of 100 points. Features that might be monitored (possibly with weight factors):

• accessing individual screens
  • Animation (= launching app from scratch)
  • Preference
  • Readme
  • Who's Who
  • Photo Clubs
  • viewing Portfolio (there are currently about 20)
• using Search feature
• pulldown-to-refresh
• changing the lock setting on a map
• viewing the end of the Readme

https://www.youtube.com/watch?v=_oTPT3gFUms