Created Issue: LoopingSelector performance improvement [10458]

This is not a bug, it is an improvement of the performances of the LoopingSelector. I changed the code in order to add next and previous items only at the first expansion with better load time of pages (i.e. DatePickerPage and TimePickerPage).

This is a detail of the changes to LoopingSlector.cs (Windows Phone Toolkit - Nov 2011 (7.1 SDK)).

1. Code added to LoopingSelector.Balance (before the initialization of firstItem, itemsBeforeCount, lastItem and itemsAfterCount).
if (!IsExpanded) return;
2. Code added to LoopingSelector. OnIsExpandedChanged (after the initialization of picker).
if ((bool)e.NewValue && picker._itemsPanel.Children.Count <= 1) picker.Balance();