[BUG] missing instance variables in GIFReadWriter ( Fix for missing vars and improvements to FileList reading )

[ned at squeakland.org]

The attached CS adds an 'offset' instVar, and also fixes/improves the
FileList handling.

* Add an 'offset' instVar to get rid of Undeclared refs.
* Open  either an ImageMorph or AnimatedImageMorph as appropriate from a
FileList.
* Allow opening GIFs from remote servers in the FileList.













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