Implement `RateReminder` feature

[nivisi]

For implementing this one, I'll go with a platform dialog. Same as with the Messages feature (#51).

NStackRateReminder will provide you with:

• NStack.rateReminder.action function
• NStack.rateReminder.shouldShow function
• NStack.rateReminder.onRequest stream
• NStackRateReminderListener widget with a callback when the request to show the reminder is displayed
• NStackRateReminderDialog widget for default platform dialog.