Improve performance of various controls and samples.

Other changes:

ItemsStackPanel (NEW)
+ VirtualizingStackPanel with Padding

TransitionFrameSlim (NEW)
+ Transition frame that uses a single content presenter instead of two

AutoCompleteBox
+ PlaceholderText
+ Tilt effect on drop down
* Fix issue where drop down was covered by system tray
* More closely match the built-in experience

DatePickerPage/TimePickerPage
* More closely match the built-in experience