Attempted to fix FlipViewIndicator when setting Items

Currently the FlipViewIndicator will only work when binding to the ItemsSource on the FlipView.

This fix isn't complete yet, the pull request is more an ask for help/discussion.

What I have currently works fine in most cases but when I start changing the visibility of the FlipViewItems this doesn't get reflected to the Indicator as it sees the item as there. I tried putting in a basic visibility check but I probably need that to run whenever on of the items visibility property gets updated.

Hoping you can point me in the right direction with this one.