Handle imagedata in test.py and validate.py

- add a check that a deleted file is not referenced via
  <imagedata href=...>
- add a check that builds a manual if a file referenced via
  <imagedata href=...> is changed

Change-Id: I34dfad7ea8d1f3a88da1b7196c49e05072908479
Merge "Handle imagedata in test.py and validate.py"