Support for `<img>` and `<video>` in SMIL

SMIL allows references to images (`<img>`) and video (`<video>`). While these two tags are not currently references by the EPUB specification, we want to make sure that we support them since they're also allowed in our model for Guided Navigation.

`<img>` in particular could be useful to create accessible comics using fragments of an image synchronized to an audio fragment.