allow for .gif screensaver images, clean-up Kobo cover screensaver images

Now that we can handle .gif in Imagewidget, we can allow screensaver
images that are stored as GIFs.

Kobo book cover screensaver images were handled a bit crude. The
document was opened three times for no good reason, and wasn't
closed afterwards either. Also, an unnecessary use of AlphaContainer
was removed.

This is untested as I'm not quite sure how to test it on emulator.