Fix for not counting launches if using Application.Current.Terminate() (+few private to public changes)

Make FeedbackOverlay.Feedback() public. It is a very useful method. 
Also, make FeedbackHelper.Store/LoadState() public and save IsolatedStorageSettings manually instead of waiting for the app to do it, because sometimes developers use Application.Current.Terminate() and in that case the app doesn't remember the launch count correctly after shutting down.