Add support for FilePicker on Windows Phone 8.1

UPDATE: The attached file contains a fix which adds support for file and folder pickers under Windows Phone 8.1. The fix contains all necessary steps but has to be refined/cleaned up.

Windows Phone 8.1 file pickers use the continuation scheme where the picked file(s) are passed back to the calling application when the application comes back to foreground.
The picked files are passed as "IContinuationActivatedEventArgs" in the "onActivated" method (see here for full source code: https://code.msdn.microsoft.com/windowsapps/File-picker-sample-9f294cba).

To support continuation, the "onActivated" method in "qtmain_winrt.cpp" has to be changed so that the "IContinuationActivatedEventArgs" are somehow stored.

HRESULT onActivated(Core::ICoreApplicationView *, Activation::IActivatedEventArgs *args)
{
	//... current code
	//NEW:
	Activation::IContinuationActivatedEventArgs *continuationArgs;
	if (SUCCEEDED(args->QueryInterface(&continuationArgs))) 
	{
		// store the args somehow?
	}
}