WIP: Add new file source element for Universal Windows Platform
Due to the strict access permission rule of UWP, only app's local
storage directory can be accessed by traditional Win32 APIs.
To read file located in arbitrary path, an app should use StorageFile
object returned by Windows::Storage::Pickers::FileOpenPicker interface.
This new element supports both uri based file open like filesrc element
and StorageFile based file open for UWP app. To make it work with StorageFile
interface, an application should pass the pointer to StorageFile interface
by "storage-file" property.CC: @nirbheek