TEI P5: Add support for <graphic> element.

[jhellingman]

The element replaces ad-hoc methods of including references to external images in P3.

• Support element.
• Support nested elements. (now non-standard element used as temporary hack)
• Make sure elements for which the related image is not available can be suppressed in generated output.
• Make sure legacy P3 files continue to be processed correctly (either directly or using a pre-processing step).