Rename `image-extension-data' to `image-metadata'.

Rename `image-extension-data' to `image-metadata'.
http://lists.gnu.org/archive/html/emacs-devel/2010-03/msg01456.html

* image.c: Add `Qextension_data'.
(syms_of_image): Initialize and staticpro `Qextension_data'.
(Fimage_metadata): Rename from `Fimage_extension_data'.
(gif_load): Put GIF extension data to the property
`Qextension_data'.

* image.el (image-animated-p): Use `image-metadata' instead of
`image-extension-data'.  Get GIF extenstion data from metadata
property `extension-data'.