Rename SwapChainBackgroundPanel

References to SwapChainBackgroundPanel are currently named
'SwapChainPanel' (and sometimes 'swapPanel').

DX11.2/Win8.1 introduces a new UIElement,  'SwapChainPanel'.
http://msdn.microsoft.com/en-us/library/windows/apps/bg182880.aspx#three

I renamed all SwapChainBackgroundPanel because having then referred as
swapChainPanel could be confusing in the near future.

Arguments become -> 'swapChainBackgroundPanel'
Properties become -> 'SwapChainBackgroundPanel {get;set;}'
fields become -> '_swapChainBackgroundPanel'