Bug fixes

Fixed the drag inertia problem by changing to using 2 ManipulationMode
parameters
Fixed the bug where when you first started dragging the item, the item
beneath it would prematurely fill where the dragged item used to be,
then correct itself (jarring flicker effect) - did this by moving any
code after the async image render in ManipulationDelta into an if
statement that made sure the image render had completed

Added some comments to the code so I can remember what stuff does